T-tail

In aircraft a T-tail is an arrangement of the tail control surfaces with the horizontal surfaces (tailplane and elevators) mounted to the top of the fin, rather than the more common location on the fuselage at the base of the fin. The resulting arrangement looks like a T when viewed from the front, hence the name.

A T-tailed British Aerospace Avro RJ-85 of SN Brussels Airlines (Belgium)

There are a number of pros and cons to this arrangement.

Pros

The tailplane surfaces are kept well out of the airflow behind the wing, giving smoother flow, more predictable design characteristics, and better pitch control. This is especially important for planes operating at low speed, where clean airflow is required for control. deHavilland Canada's line of larger STOL aircraft all use this arrangement for this reason.
The effective distance between wing and tailplane can be increased without a significant increase in the weight of the aircraft. The distance between the two planes gives the "leverage" by which the tailplane can control the aircraft's pitch attitude - with a greater distance, smaller, lighter tailplanes and elevators can be used.
The tail surfaces are mounted well out of the way of the rear fuselage, permitting this site to be used for the aircraft's engines. This is why the T-tail arrangement is also commonly found on airliners with rear-mounted engines. The Douglas DC-9, Boeing 727, Vickers VC-10, Hawker-Siddeley Trident and BAC 1-11 all used the T-tail for this reason.

Cons

The aircraft will tend to be much more prone to a dangerous deep stall condition, where blanking of the airflow over the tailplane and elevators by a stalled wing can lead to total loss of pitch control.
For similar reasons, T-tailed aircraft can be much more difficult to recover from a fully-developed spin.
The fin must be made considerably stronger and stiffer to support the forces generated by the tailplane. This inevitably makes it heavier as well.
The control runs to the elevators are more complex.
The elevator surfaces are much more difficult to casually inspect from the ground.

Some aircraft feature a tailplane that is mounted part way up the fin rather than right at the top. This form of design combines some of the pros of both the T-tail and conventional arrangements, while avoiding the most serious cons. The Sud Aviation Caravelle is one example of an airliner with a mid-fin mounted tailplane.