beta is apparently more than a fish and a Greek letter

Are there any readers of my blog that can explain to me why I keep seeing things advertising a beta version? What does that mean exactly? Blogger was in beta and now they're all happy about not being beta anymore as though it's a great accomplishment or something. The only change I can see is that now I can (for the last three posts) post to my blog from home. Yahoo email also has a beta version. I've seen it in other software, as well. What does it mean?!