BMW last used the 's' for the E36 325is, which ceased production in 1995. Revived for the 335is, the letter indicates a sport-tuned version of the 335i that along with more horsepower, gets the option of BMW's seven-speed dual-clutch transmission, an option not available in the 335i.
The 335is' twin-turbocharged 3-liter inline six-cylinder engine generates 320 horsepower and 317 pound-feet of torque. This engine is a holdover from the 2010 BMW 335i--the 2011 335i gets a slightly different engine.
This iPod interface is poorly constructed, as each category serves as a filter. Choose an artist, and the album category will only show albums by the previously selected artist, not something you want to mess with while driving.