First, Bitch means a "female dog". I think everyone knows that.
Then as an insult, this was thrown at some unfortunate woman in the long lost past and it stuck. So, for many years, it was a pretty bad insult that might get you a slap in the face if you called someone it openly. If you were bitching about something, then you were really complaining.
And recently, well, it's now something else. There are those that say the word in a jaw dislocating way so that it's now "bee-atch", which I find fairly tiresome. It's taken on a whole S&M subservient slave connotation. It's a word of power, if someone is your bitch, then they do what you tell them to. But between friends, it's seen by some as a conspiratorial term of endearment, like you are a party-animal or something, or perhaps a bit moody.
And on blogs of late I have seen it as replacement, almost, for the word "everyone". It is an exclamation after a word or phrase, to add emphasis. For example, "Gold... bitches!"
I'm over all of it. I think the word has peaked. I'm hoping it will recede into the background as quickly as it's risen to prominence - like "bad" (meaning good) or "whatever" (meaning I don't care). Mind you, if it does this it will probably because it has been replaced by something even more irritating. I don't know what it will be exactly, but my kids like to use "random" instead of bad (meaning bad). That's...er.. fairly random, so whatever, bitches.