The aerodynamic forces of the airplane's wing produce lift but also produce a moment that tends to pitch the aircraft nose down. This can be corrected by designing the Wing such that the Center of Lift is forward of the Center of Gravity.
The Tail acts as a small wing that allows the pilot to control the pitch of the aircraft through the various flight regimes. The Horizontal Tail is a wing that also produces lift. On many a/c, this lift actually acts DOWN so that it counters the Nose down pitch induced by the a/c Wings.
T-Tails are horizontal control surfaces that are mounted on the top of the Rudder of the a/c. This design places the tail surface out of the turbulent flow that sheads off the a/c fuselage and propellor. The disadvantage is that during height angle of attack, such as landing, the flow off the wings can suddenly wash over the tail surface and kill its effect.
This is a serious problem. The old Fokker Dr.1 Triplane had 3 main wings and the tail was a large triangular surface. During landing the entire tail surface would stall and the pilot would loose complete control effects of the tail.
Most large commericial a/c have the ability to trim the tail to balance out the pitch of the a/c. This is done by adjusting the angle of attack of the horizontal tail surface---the whole tail moves.