Trim levels differ by features. Lower trim levels are more bare bones, while higher-trim-level vehicles are upgraded with more bells and whistles.
For the Lexus IS, trim differences are most visible with regard to the the engine. The IS 250 is equipped with a 2.5L V6 engine capable of 203 horsepower. This is pretty powerful, but it’s nothing compared to the 350, which has a 3.5L V6 engine that can produce 311 horsepower. If you’re hoping for a more powerful vehicle, the 350 trim is definitely the one for you.