iPod's are not always identified by "generation." So if your question is about the
G of your iPod...
For the full size iPods:
1G is the +iPod with scroll wheel+. It has the wheel that physically turned, and four buttons around the wheel (plus the center button).
2G is the +iPod with touch wheel+. It looks very similar to the 1G except the wheel is now touch (no longer turned).
3G is the +iPod with dock connector+. It has all touch controls, including the four glowing buttons and the center button. The first model with the dock connector.
4G is the +iPod with click wheel+. The first model with touch wheel that also clicked down for the four
button functions. There are actual two distinct types - one with color screen and one with mono screen.
5G is the +iPod video+. Apple actually identifies it as "Fifth Generation." The first model that played video.
6G is the current +iPod classic+.