A resistor is an electrical component used to provide electrical resistance in a circuit - that is, it impedes the flow of current. Resistors are usually made of carbon and marked with coloured bands which indicate the level of resistance they provide (measured in ohms). Another kind of resistor is a potentiometer, in which the resistance is adjustable.
Unlike inductors and capacitors, which (in theory) don't dissipate power, a resistor dissipates power, equal to the product of the voltage across it and the current passing through it. Therefore, resistors have a power rating, and some resistors can be quite large in order to dissipate the required power (as heat).