Sims 3, is like many of the Sims games. It focuses on your characters goals, determined by their traits (Which if you make your own character called a sim you can pick). The difference between Sims 3 and the ones in the past is:
-You can choose traits for your sim
-You have a life time goal
-For accomplishing goals you get lifetime happiness point
-You can purchase perks (For your sim) with these points
-PC version has expansion pack with new items, clothes, make-up, and supernatural/pets.
-You can travel around town and to other's houses as you wish
-Enabled aging (Your sim can go from a baby to an elder and die)
-With the aging your sim can also get pregnant and have children of their own.
-There are new social options/types of relationships/ect.
-With game updates and such your sim also has a scrapbook full of memories and a bio
These are all interesting features introduced into the game. Though but with Sims, its a different kind of game, some people love sims and others don't Sims 3 is a Sims game, you don't venture out into the world and fight monsters, there is no set story line. You should know all this, even if its just from hear say alone. The only problems I see with Sims 3 is, they took out the whole 2 player option, it can not be played in two player, not even on console games such as Xbox and PS3. Which takes the fun out of it a bit. Also, the many expansion packs can get a bit costly if you want to keep getting new things. Over all, I am a Sims fan and the game on PC is actually quite entertaining especially with the expansion packs.
Warning there are some problems with the game freezing that have been reported, and I have experienced it as well.