I've been looking at buying a used ipod touch from craiglist but I'm confused as to what's what. The adds all say things like "2nd generation 8gb" or "3rd generation 16gb" and I'm not sure what the difference is.
The short answer is processing speed, screen resolution and memory. The newer ones are obviously faster, and the lastest generation (4g) includes both front and back cameras for shooting video and video calling.
For a full list of the differences check out this wikipedia page.
The long answer:
1st and 2nd have 128MB RAM while 3rd and 4th have 256.
3rd and 4th also come in 64GB versions as well as 32GB. The 4th added back an 8GB model.
2nd added Bluetooth (Only in iPhone OS 3.0)
3rd added voice control.
4th also has a Retina Display. Though not the same quality as the iPhone 4. Still it's nice and high-resolution. (It's an LCD, not an IPS for price reasons.)
And the 4th has two cameras as well. Still not as nice as the iPhone ones.
Processor is as follows: 412MHz, 533MHz, 600MHz ARM processors in first three versions. (Underclocked speed)
4th generation replaced the processor with an Apple-made 1GHz A4 processor underclocked to 800MHz.
4th also added a microphone built-in and a 3-axis Gyroscope.
The 5th generation will be released in September. This is 99.9% guaranteed. Most likely with some features from the to-be-announced iPhone 5. (iPods are always September every year. It's one of the only predictable things about Apple. iPhones are in June or July when WWDC takes place, but can change without notice.)
I could keep going but then I'm just into the useless techy stuff like GPU and weight.