It is hard to describe either because they are moving targets. Both games are in beta and genuinely are being updated with new features regularly. What is more, they tend to copy each other's improvements and additional features. One of the biggest differences is that Farmville, by Zynga, like all their games tries very hard to get you to pester lots of other people into playing. You can't expand your farm without lots of "neighbors". In Farmtown you get advantages from neighbors but you don't have to have them. Another difference is a genuine social aspect to Farmtown. You can get other players (not Facebook friends, just other folks in the game) to help on your farm. You can also chat and socialize with them. This aspect does not exist in Farmville. One feature that Farmville has to itself is that farm animals are useful and part of the game. In Farmtown the animals are just decoration. Both games have recently added farm machinery to make the game mechanics less cumbersome. The implementation in Farmtown is much better and is a huge boost to the playability of the game.

The one difference I enjoy in FarmVille is the ability to gather wool from my sheep and gather eggs from my chickens. In FarmTown you simply had animals wandering around your farm making you buy fences to corral them. I enjoy both games, but I like FarmVille a little more for these reasons.

If you like those games you should try FishVille it is a fun aquarium game. Brand new and very fun.