To explain evolution, Darwin talked about two things

1)   Branching descent
2)   Natural selection

Branching descent explains the ‘pattern’ of evolution and natural selection explains the ‘mechanism’ of evolution. Branching descent and natural selection are the two key concepts of the Darwinian theory of evolution.
Branching descent: He said that evolution is a branching process. With time, new life forms appear and these new life forms arise as ‘branches’ from the previously existing life forms. Hence, branching descent points towards the concept of common ancestry.
Natural selection: He emphasized that the selection of ‘useful variations’ by nature is the main (but not exclusive) mechanism for evolution to occur.