Resistance applies to both d.c. and a.c. circuits, and is determined by the resistivity, length, and cross-sectional area of a conductor.
In d.c. circuits, resistance is the only opposition to the passage of current. However, in a.c. circuits, the flow of current is opposed, not only by resistance, but also by reactance. Reactance is caused by a circuit's inductance or capacitance, or both, and varies with the frequency of the supply. Like resistance, reactance is also measured in ohms.
So, in a.c. circuits, the combination of resistance and reactance is called 'impedance'.
Impedance is not the algebraic sum of resistance and reactance, but the vectorial sum. So if, for example, an a.c. circuit had a resistance of 3 ohms and a reactance of 4 ohms, the impedance would be 5 ohms, not 7 ohms.