Is there a difference between a (regular) starter and a sourdough starter?

All sourdough starters are natural starters but not all natural starters (or Poolish - as mentioned below, are sour. The Handmade Loaf by Dan Lepard is the best book I know for learning a myriad of techniques, from all over Europe, for making bread of every kind - natural yeast to quick bread.

To be clear, a "sour" starter is one that has gone off. Controlling that "sourness" is tricky. It is said that there is more wild yeast in the air in California and Germany, but wild yeasts of all kinds exist everywhere. And that is the beauty of making bread from a starter - it's always a bit of hope, alchemy, magic and science that makes your next loaf come out!

There is a maize bread in Dan Lepard's book that calls for starter & yogurt (I used Labne) that is gorgeously sour and dense. I could not stop toasting and eating it when I had it in my kitchen!

There is a huge equivocation (same word with different meanings used interchangeably) with the word 'starter' that is very common in bread making. What makes it worse, is this is a method that has been in practice (in one form or another) for several thousand years. Between regional variation and the changes over time, it's hard to find a single unified definition for this word, but I'll try anyway.

Starter: these days, generally means the place where the yeast is living, usually a flour water mush.

Starter can be from cultured sources (ie you buy the yeast and then give it a place to live) or from wild sources (you catch and tame the wild yeast from the air around you). I find they take about the same amount of time to get growing and the culture starter is usually overwhelmed by your local yeast in a few months, unless you take steps to prevent this

One of the biggest things that gets my goat is how complicated people try to make sourdough bread. It's not. HEY PEOPLE, this is the simplest bread in the world. You want a fast making bread, you want a slow making bread, you want it sweet, you want it sour, you want something you are going to fuss hours over, or you want something you can spend 10 min on, forget for two days, then bake a fantastic bread out of - that's what sourdough is. It quite literally is the people's bread and it will conform to your schedule and with very little effort on your part, will make the bread you want. Okay, rant over.

Catching a starter: Since the yeast and bacteria (yes, there is bacteria in this, it's good for you and helps break down the starch to feed the yeast) are already living all around you, the simplest way is to mix flour, water and nothing else, into a porridge mix. Thick and thinness doesn't matter so much at this stage, but easy to stir is best. You can stir it daily or not, but best keep the flys out of it. After about 4 days it should be bubbling, if not, then add a few more Tbs of flour and water, give it a good stir. Give it a few more days. Failure: toss it out, change a variable (flour, water, location) and start again. You can add many things to a starter to get it going, I've tried lots of them, most don't make things go any faster, in my experience, but it does make people feel like they are doing something useful, and that's worth a lot. If you do want a recipe that adds extra, try Nigella Lawson's book How to be a Domestic Goddess - great sourdough starter recipe and information there.

Despite popular opinion, not all yeast likes the warmest part of your kitchen. I had a devil of a time catching a starter in my new house, until I mixed up some new flour/water goop and put it in the coldest part of the kitchen, 6 hours later, very active starter, no other variable had changed except location.

Probable causes of failure to catch a starter: ANTIBACTERIAL SOAP IS YOUR ENEMY. You're trying to grow bacteria and yeast... this soap is designed to kill bacteria and yeast... some sort of link here that isn't quite clear to people... hmmm... Another probable source of failure, that stuff the city puts in your water to keep you safe from bacteria and other harmful invisible nasties. Solution, boil and cool you water before use. Third most common problem, the processing in flour to prevent spoilage by things like bacteria and yeast, or the bleaching which makes it white, but also kills bacteria and yeast... there is a theme developing I think.

How thick to keep your starter: Thick = sour but lasts longer (both starter and the finished bread), thin = sweeter and loftier, but needs feeding more often. There is a lot of info out there about hydration rates, this is a wonderful tool if you want to make exactly the same bread, every single time, hundreds of time a day - oh, like a bakery. This is also great if you are highly technical (I am, I love it) but pointlessly confusing if you are just a normal human wanting to make yummy bread.

Now the Sponge (sometimes called poolish but these days Poolish is often use to refer to a sponge that is made with commercial yeast but acts the same as a sponge): This is a mixture of starter, flour, water, and other things like leftover cooked grains. It's wonderful when you keep your starter in the fridge as it helps wake up the yeast and gets it ready for tomorrows baking. This is another stage where you can control how lofty or sweet/sour your bread is. The thickness of the starter has a lot of influence on the shelf life of the finished loaf (I keep an almost solid starter and my bread lasts for 3 to 10 weeks at room temp, in plastic, before it moulds or goes hard), the sponge can take a very sour starter and make a very sweet bread. Thinner sponge = sweeter bread, thicker sponge = more sour bread.

You have no need whatsoever to make a sponge even if you are starting from very cold fridge starter... it does improve the loft of the bread however, and speeds up the actual rise time. However, making a sponge takes time as it needs to sit a few hours to activate the yeast... so roughly the same time total as not using a sponge.

This is getting long, so I won't go into dough, how many rises, how to make the timing fit your lifestyle instead of becoming a slave to your bread, baking, shaping, and how to cool it for crust texture and shelf life.

Basically the points I want to make are:
1) sourdough starter is a term that has had many meanings over the years and location, it's been around a bit, so one would expect this. If you are reading a book, don't expect them to use the same meaning as the other book, and unless they state otherwise, they often don't use the same definition throughout the book.
2) it's easy, simple, adjustable, wonderful bread that people have been making (in one form or another) for at least 6000 years. Probably a good deal longer. Everyday people, who didn't have measuring cups, easy access to scales, hydration meters... all that lovely modern stuff. . . basically if you don't want it to be complicated, ignore people who say there is only one right way to do things.
3) if you want to make sourdough complicated, by all means do so. Some of the most fun I've had with it have been playing with the different techniques and tools... just don't impose it on others too strongly in your enthusiasm, thankfully no one here has yet but I've seen it in other groups. Sourdough should be accessible to everyone and we forget that all those fun toys and techniques are really intimidating to beginners.

Have a look at the library for Wild Fermentation by Katz for a much more free form approach to sourdough, and the before mentioned book by Lawson. Also, there is a tiny booklet called Baking with Sourdough by Pitzer out there somewhere, that is fantastic.

A starter is the term used for flour and water that has become colonized with wild yeast and bacteria. You can make a starter yourself from flour and water - and patience - either with or without other ingredients that accelerate the colonization (like the grape skins mentioned above.) Or you can just borrow a small amount of starter from someone else. Either way, the culture is maintained by perpetually discarding most of it and then "feeding" the remaining starter with flour and water.
In contrast, a mixture of flour and water and commercial yeast is not called a starter. There are recipes that call for commercial yeast to be mixed with small amounts of flour and water and then allowed to "pre-ferment" before being added to the dough. This pre-fermented mixture is called a poolish. Some recipes - like the baguette dough in Tartine Bread - call for the addition of starter (or levain, in French) as well as poolish.

A true sourdough starter is predicated upon wild yeast spores which are obtained by first soaking either fresh organic grapes or organic raisins in warm water, then adding whole wheat flour, which is also a good source of wild yeast (known as saccharomyces exiguus). The starter, or barm, is then fed combinations of bread flour and water over a period of a few days in order to grow the yeast population. A sourdough starter is a bit more acidic than a starter based on commercial yeast (saccharomyces cerevisiae) in that it naturally grows both lactic and acetic acids, hence the creamy and tart, or sour, flavors and aromas.