A throttle body is the part of the air intake system that controls the amount of air that flows into an engine's combustion chamber. It consists of a bored housing that contains a throttle plate (butterfly) that rotates on a shaft. When the accelerator (gas pedal) is pushed down, the throttle plate opens and allows air into the engine. When the gas pedal is released, the butterfly closes and effectively chokes-off (throttles) air flow into the combustion chamber. This process effectively controls the speed of the engine and ultimately the speed of the vehicle.
As for Tappets, a tappet is only that part of a rocker arm which makes contact with an intake or exhaust valve stem above the cylinder head of an internal combustion engine. As the cam rotates, it creates both a sideways and a downward force on the tappet. Without a tappet (and with the cam acting directly on the valve), the sideways force would cause the valve stem to bend. With a tappet, the sideways force is transferred to the cylinder head so only the downward force acts on the valve stem.
Tappets/lifters are engineered to last the lifetime of the engine when the engine is properly maintained, including proper oil and filter changes. Though they are reliable, they have their drawbacks; valve clearances periodically require adjustment as the contact surfaces of both tappet and valve stem wear.
Source: Tappet - Wikipedia, the free encyclopedia
I hope that clarifies your question.