self-rising flour verses plain flour?

You can read all about it here in my piece about Chemical Leaveners on this very site! :
http://www.food52.com/blog/3130_on_chemical_leaveners

But, in summary self-rising flour is just flour with baking powder and salt already added to it (1 1/4 teaspoons of baking powder and 1/4 teaspoon of salt per 1 cup of flour). Easy peasy!

You can read all about it here in my piece about Chemical Leaveners on this very site! :
http://www.food52.com/blog/3130_on_chemical_leaveners

But, in summary self-rising flour is just flour with baking powder and salt already added to it (1 1/4 teaspoons of baking powder and 1/4 teaspoon of salt per 1 cup of flour). Easy peasy!

Just going to summary what's already been said. These are guidelines for written American recipes:

flour = plain flour. American recipes will specify if it's self-rising flour (self-raising flour in US) or any other type of white flour (cake flour, bread flour) that is not plain flour. So if you see just 'flour', it means plain flour. When to use will be defined by the recipe. Recipes using plain flour will have some sort of leavening (like baking powder or acid/base combo) listed in the recipe.

One big thing to note (because I find that people are confused about this too), cake flour is NOT self-rising flour. In the US, cake flour is a plain flour with a low percentage of protein (low protein means that foods baked with cake flour will be lighter with a more tender and delicate crumb than those made with all purpose flour). Again, a recipe will usually list cake flour specifically and not just list 'flour'.

One caveat about using self-rising flour is that the leavener will lose its potency over time. Also,
several brands of self-rising flour that have a lower protein content than all purpose flour (11% protein). They are effectively cake flours (8% protein). Wheat protein, or gluten, is what gives baked goods much of their structure, but it can also cause a bread to be too dense or tough. White Lily and Presto are two examples of self-rising brands that use a low-protein cake flour as their base.

I would suggest that if a recipe (especially here in the USA) does not call for a specific flour, then use AP flour. In the UK it (used to be, and I presume still is) that SR flour is quite often used. Recipes will often use the designation Plain flour which is roughly comparable to US AP flour. Bread flour in the UK is sometimes labeled as strong flour. And that's just white flours. Dealing with the various wheat desigbations need an advanced degree in semiotics!

Self-rising flour has baking powder and a little salt added in. It's used for baking, and is often in British recipes. When a recipe calls for flour it generally means All Purpose, or plain flour.