An NPN transistor is one that is made up of three layers of semiconductor material, a thin N-type layer (which the base is connected to) sandwiched by two thicker P-type layers (the collector & emitter), with the emitter being heavily doped. Whenever a small current flows into the base, the P-N junction of the emitter & base is forward biased so electrons start moving from the emitter up to the base. Now the majority of these electrons continue to move across to the collector, while a small number exit through the base. Note that the ratio of electrons leaving through the collector, to the electrons leaving through the base is constant, and this determines the gain ratio of the transistor. Because alot more electrons leave via the collector rather than the base, the collector current is significantly higher than the base current by a constant factor. The typical voltage drop across the base & emitter of a silicon transistor is ~0.6V
What a transistor does is when a base current flows through it, a larger current going through the collector is caused to flow. When there's no base current, no current can possibly flow through the collector and emitter of the transistor because it is like two diodes put back to back, effectively blocking the current. The fact that a larger collector-emitter current can be controlled by a small base-emitter current is extremely useful for amplification or switching purposes. The benefits of using a transistor rather than a mechanical device such as a relay is that it's more reliable since there's no moving parts, consumes less power, has a faster switching speed, and the capability to amplify not just switch on & off.
Transistors are far from useless, they're used everywhere especially in computers and logic circuits. Transistors form the basis of TTL (Transistor Transistor Logic) technology for making logic gates.
For some further reading check out this site:
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html
