What is an heirloom tomato
When anything is referred to as "heirloom" it means the seeds can be traced back genetically uncrossed and unmutated for at least 50 years. That's why you see all the pretty different colors from the standard red tomato because the genes of those particular seeds haven't been altered to only yield the red color that we're all used to. It also means, as Diana mentioned, that they taste much more wholesome and tend to be more nutritious.
Something you shouldn't consider purchasing until summer time.
It's winter now, and all tomatoes are shipped from southern hemisphere to US markets. Canned tomatoes are option now, or hot house tomatoes and cherry tomatoes are best for 'fresh' tomatoes until local sources produce local tomatoes in season.