Diodes, the most simple semiconductor devices, function as the electronic version of a check valve - they allow current to flow through them in one direction, and block current from flowing through them in the opposite direction. This is accomplished by creating an electrical junction between a p-type semiconductor block and an n-type semiconductor block. See also semiconductor, transistor.
There are other types of diode, that have a similar function, of only allowing electrical current to flow in one direction.
Point contact diode. This works the same as the junction semiconductor diodes described above, but it's construction is simpler. A block of n-type semiconductor is built, and a conducting contact made of some group-3 metal is built to a sharp point which is placed in contact with the semiconductor. Some metal migrates into the semiconductor to make a small region of p-type semiconductor near the contact. I'm not sure if these are still used much. They were fairly widely used for small power applications and digital electronics circa 1960 to 1980. They were not so popular in analogue applications due to high noise and non-linearity.
Tube or valve diode. This is the simplest kind of Vacuum tube device (refered to as valves in the UK). These are probably completely obsolete by 2001, but for much of the 20th century were used in analogue signal applications, power supply applications, and (approx 1950 to 1960) for digital electronic logic.
Gas discharge diode. There are two electodes, not touching, in some kind of gas. One electrode is very sharp. The other has a smoothly curved finish. If a strong negative potential is applied to the sharp electrode, the electic field near the sharp edge or point is enough to cause an electrical discharge in the gas, and a current flows. If the reverse potential is applied, the electrical field strengh around the smooth electrode is not enough to start a discharge. (The discharge can only start easily at the negative end because electrons are much more mobile than positive ions.) These are sometimes used for high-voltage high-current rectification in power supply applications.