Answer:
Impedance is voltage divided by current. This is often referred to as resistance as well. Impedance includes resistance and reactance; in a DC circuit, this is an unnecessary distinction.
Reactance can be capacitive and/or inductive, and is a function of the AC frequency.
Resistance and impedance are different, because impedance includes reactance as well as resistance; If you are working with DC voltage, this becomes an unnecessary distinction since the reactance of (ideal) inductors is zero at zero frequency (DC voltage). Reactance of (ideal) capacitors at DC is infinite, so capacitors are taken to block DC, and thus it is unnecessary to handle any differently than to say capacitors block all voltage and treat as an open circuit. Capacitors are used in amplifiers for this purpose, and generally are removed from the circuit (an open circuit is left in place of the cap) when analysis is necessary (DC analysis).
In the final analysis, Ohm's law prevails: Impedance (resistance) is volage divided by current, so long as you look at it correctly.