Car not starting: Cranks for awhile but never starts then it makes a very sad noise :(

I'll try my best to explain what is happening but I don't know much about cars. I took a video so you can hear the sound it is making: http://www.youtube.com/watch?v=Bd-gNEROEPU

This sound is new. Lately it been cranking, but not starting. If I crank it for awhile it will eventually start but today it never did and eventually it did that pitiful noise in the video (I really don't know how to describe that noise so please watch the video and listen carefully!).

It's a 2002 Nissan Sentra 1.8 cyl and I was kinda of low on gas yesterday so maybe that has to do with it? Any idea what the deal is? Just trying to figure out if it might be something I can fix (or a friend can fix ;) ) or if I need to somehow get it to a shop.

If you need more info, let me know! Thanks!

Update: Battery was dying so we charged it, now it's doing this: http://www.youtube.com/watch?v=D4x0qJIEnTE