What do the numbers and letters refer to in camera names?

Beginner question, I know. I really want to expand my knowledge of photography, so I thought I would find out the answer to this. I'd prefer no condescending or sarcastic responses, I'm only an amateur wanting to learn.So, my question being, what do all of the numbers and letters mean in camera names? Ex. Nikon D5000, Canon Rebel xsi, etc.etc.
Bonus question, do you prefer Canon or Nikon and why? Compare settings, which ones are superior to the other?
Thanks so much in advance, please write a bit and explain.

The D stands for digital, the numbers are random and mean nothing really except they are replacing the last model - eg d200 is replaced by d300 or 50d ie replaced by 60d - its not like BMW for example a 320 is a series 3 (body size) and a 2.0 litre engine, a 325 has a 2.5 litre motor et cetera…

- argueing over nikon vs canon is way below me, they are japanesse cameras that is all, they are the same to good fotogs…

Yep, "d" is for digital. After that, the numbers and letters mean nothing. Canon and Nikon are like Honda and Toyota: similar features, functions, prices. The one you prefer, believe me, will be the one you own.