The side channel attack is based on the fact that the time difference between certain operations executed by the implementation of a cryptographic scheme allow an adversary to get information that he didn't have according to the theoretical specification of the scheme.
Timing attacks analyze the speed of implemented algorithms.
At the hardware and software layer, complex operations take more time than simple operations. Furthermore, the dataflow can be analyzed because access of CPU registers, cache, and memory have different timing characteristics.
Timing attacks are related to power analysis.
- Timing attack on Wikipedia