What EXACTLY does it mean to be "judgmental?"?

I mean, you hear it all the time. "They're so judgmental!" or "Don't judge me!" I understand the concept, being judgmental = judging somebody, but how does that play out from a practical standpoint? What about disapproving of something, are you judgmental when somebody is doing something you don't agree with? I once speculated whether or not a stranger was gay (just an innocent observation!) and I my friend said I was judgmental. What did I do? I have pretty good gaydar, and I thought maybe he was... I've never really understood it.