WHAT ARE GATES?
“Gates” are another name for transistors. Transistors come in a variety of configurations and serve a variety of purposes. Some keep the flow of electricity going in only one direction while others permit electricity to flow only under special conditions. Why, one might ask, does anybody need these qualities? If we look at a variety of conditions, we may see.

Suppose you are the floor supervisor of a perfume factory. Suppose one position causes you a great deal of trouble. The person who puts the bottles in the boxes typically gets tired of the job after only a few days and quits after only a few weeks. You want to automate.Next

The three examples above represent only a few examples of the dozens of sizes, shapes, and capabilities found in transistors. Some transistors direct electrical current under special conditions; some shed light, others boost power. In the past fifty years they have evolved from a novelty to a critical tool for our daily lives.

Intel has produced an excellent description of how transistors work.

Lucent, originally Bell Labs and inventor of the transistor describes how transistors came to be.