Probably talking about the Core i3/5/7 processors. First gen was Nehalem, second gen Sandy Bridge and third gen Ivy Bridge.
Most drastic changes are between first and second gen. Sandy Bridge added QuickSync, which is pretty much the undisputed king of video encoding (where supported) and the HD graphics engine, which allows you to skip a discrete card for low end builds not oriented toward gaming.
Aside from that, second gen is quite a bit faster than the first gen, and overclocks extremely well. Third gen is about 5%-8% faster than second gen and packs a faster HD graphics engine for slightly better gaming (still not a good use for it) and even better encoding. Third gen runs much hotter than second gen, however, so it doesn't overclock as well without exotic cooling.