W/T is the base trim. The pillar emblem will read 'Silverado'. This trim will be basic; crank windows, manual locks and mirrors, vinyl seats with a cloth seating surface option, shift on the floor 4x4, vinyl floor.
LS is the mid level trim which will afford you all the power options (windows, locks, mirrors), full cloth seats, carpet, rear defogger, optional power seats, leather wrapped steering wheel, and push button 4x4.
LT is the top trim which will add a full center console, optional power leather seats with heat, heated mirrors with signal, Bose stereo, automatic climate control, and steering wheel controls.
There are other differences, but these are the basics to give you an idea.