A beta (or beta version) is a term used to describe a version of software which is not finished yet. More specifically, it is usually a version which is shared with a small number of users for testing. The idea is that real users will inevitably turn up more issues than any test script will, and thus it is good to test products with a, not insignificant, number of users.
Beta’s are often shared too early and too widely by over enthusiastic founders and evangelists in an attempt to use them as pseudo marketing tools. This can lead to statements such as:
“Yah, I know it is buggy as hell and you can’t sign in and when you press f5 the world sets on fire, but it’s a beta though so that’s all ok. First to market, yo.”
Weirdly, although people bang on about their beta version constantly, they rarely talk about their alpha version. The alpha really is just the live product that in theory doesn’t make the world explode or make your grandma cry.