There are several updates that Nikon did to make the D5100 stand out a little more over the original D5000.
First the differences:
They both have:
http://nikonites.com/d5100/2499-d5100-vs-d3100.html
http://nikonites.com/d5100/2500-d5000-vs-d7000.html
http://nikonites.com/d5100/2507-d5100-vs-d90.html
http://nikonites.com/d5200/9382-d5200-vs-d5100.html
First the differences:
They both have:
- 11 AF Points
- 4 frames per second continuous shooting
- No in-body focus motor
- CLS compatibility, but no commander mode
- Plastic bodies
http://nikonites.com/d5100/2499-d5100-vs-d3100.html
http://nikonites.com/d5100/2500-d5000-vs-d7000.html
http://nikonites.com/d5100/2507-d5100-vs-d90.html
http://nikonites.com/d5200/9382-d5200-vs-d5100.html