A century is 100 miles or possibly 100 km (usually refered to as a metric century by US folks)
A time trial (or TT, but be careful of the context because tt can be short for "top tube" as well. Also can be ITT - individual time trial or TTT - team time trial) is a race where each rider or team goes off one at a time on a set course and tries to ride the fastest time. Individual Time Trials are often called the race of truth, because unlike in a pack you don't have any help, there is no hiding from the wind and no team mates to pull you. Very short ITT's held at the beginning of a race are often called Prologues. Local level times trials usually run around 10-12 miles in length, our state level ITT was a 40K.