In series
The combined resistance is equal to the sum of the resistances
The current is the same at all points around the circuit
The potential difference of a power supply is shared between the resistors i.e. V = V1 + V2 + V3
In Parallel
The effective resistance is less than the resistance of either resistor, beacuse it is easier for the current to flow i.e. 1/R = 1/R1 + 1/R2 + 1/R3
The current from the source is greater than the current through either resistor, beacuse the current splits, i.e. I = I1 + I2 + I3