An electric current flows when electrons move through a conductor, such as a metal wire. The moving electrons can collide with the ions in the metal. This makes it more difficult for the current to flow, and causes resistance.
The length of a wire and its thickness affect the resistance in the circuit. The longer the wire, the greater the resistance because electrons collide with ions more often. The resistance of a thin wire is greater than the resistance of a thick wire because thin wire has fewer electrons to carry the current.
Resistance can be found by measuring the current flowing through it, and the voltage across it. The equation for resistance is R = V/I where R is measured in Ohms, V in Volts and I in Amps.