Inverters and gates are combinational logic devices because they merely combine inputs according to their logic function and output the result. Time is not really involved. Sequential logic devices have memory or states. 

A flip-flop is a basic memory device that can store one bit of information. The simplest "flop" can be made of two inverting gates. NOR and NAND flops are shown below. For the NOR-implemented flop, assume that the Q output is 0 and /Q = 1. (The "/" is "NOT" so that /Q is the logic inverse or complement of Q.) The flop is in the reset state. Assume also that the S (set) and R (reset) inputs are at 0.

The S input logic level now changes from 0 to 1. This causes the upper NOR gate output to change to 0, so that /Q = 0. The /Q input to the lower NOR gate allows its output to change so that Q = 1. The flop has changed state, from Q = 0, to Q = 1. The Q input to the upper gate forces its output to remain 0 so that the S input no longer matters; it can be 0 or 1 and the flop state remains. The flop "remembers" the state it changed to regardless of the subsequent S input. Consequently, only a high pulse on S causes the flop to be set (Q = 1). A high pulse on R will now cause the flop to change state (be reset) so that Q = 0.

The NAND-gate RS flop, works similarly, but with low-going input pulses. By alternately applying low pulses to its S and R inputs, the flop alternates between states.