Transistors
Overview
Transistors are 3 legged active semiconductor devices. Check out the Bipolar Junction Transistors (BJTs) page if you are after the classic transistor which is current controlled and has a base, collector and emitter. Check out the Metal-Oxide-Semiconductor Field-Effect-Transformer (MOSFET) page if you want to learn more about the more modern transistor which is ubiquitous with CMOS technology which is used in almost all ICs and CPUs today. MOSFETs are voltage controlled and have a gate, source and drain.
For more exotic transistors, you might be interested in the junction-gate field-effect transistor_ (JFET) which is like a MOSFET but has a gate current, and is used for things such as low-noise op-amps and simple current sources.
Silicon controlled rectifiers (SCRs) are a form of thyristor which can switch large currents and latch on until the current reduces to zero.
Transistor Part Number Standards
If you are lucky enough, the transistors you have will follow a naming standard. This allows you two identify (or partially identify) a transistor from bits of it's part number.