In series circuits, there is only one possible path for the current. The current is the same in every position and the sum of the e.m.f.s equals the sum of the IR products. In parallel circuits, there is more than one possible path for the current. How much charge flows down each path depends on its resistance (more resistance means less current).
The total resistance R of two or more resistors in series is given by:
The total resistance of two or more resistors in parallel is given by: