In my limited understanding...
It is a handheld device, basically a palm sized computer. You can get on the internet w/ it. It has a tiny keyboard on it. You can send and receive email and still use it as a cell phone. It also holds information, your contact list, etc. You can also sync it to your calendar on the computer. And if it's like a palm in this regard, you can transfer information to another blackberry. And in my opinion, yes they are expensive, several hundred dollars.