TK is the suffix for a Stormtrooper. It comes from the section in ANH when two of the troopers are guarding the Millenium Falcon, and they get ambushed by Han and Luke. One of them comes back out and you hear the officer saying the immortal line "TK-421, why aren't you at your post, TK-421 do you copy?". When the 501st was formed Albin decided to use the designation TK for all Stormtroopers. TB is the designation for a Biker Scout, TI is for Tie Pilot etc.
Hope this helps
Martin