Semiconductors have revolutionized the field of electronics and have directly influenced our lives. Computers and almost all other computing devices today are semicondutor based. What makes semiconductors so useful is the ease with which the flow of electrons can be controlled in semiconductor devices. The number and direction of flow of electrons (or other charge carriers) can be easily controlled by making a junction of two different semiconductors and then applying external influences like a voltage, light or heat. These pages will describe how.
Basically, semiconductors are substances that have electrical conductivities in between those of metals and insulators. This difference in conductivity of semicondutors is due to structural differences. These structural differences also lead to a mechanism such that there is an increase in conductivity with increase in temperature. This behavior is opposite to that of a metal. Also, the conductivity of semiconductors can be altered greatly by addition of small amounts of other substances in them.
These and some other interesting characteristics of semicondutors can be understood on the basis of the principles of quantum mechanics. Here you'll find some of these explanations. Although you do not need to understand quantum mechanics too well to get familiar with semiconductors, if you wish, you can see the section on Quantum Mechanics.
To get started with learning about semiconductors, their interesting properties, why they behave the way they do and about semiconductor devices, see the following links...