Git Merge and Rebase

There are two ways you can join branches.
For showing with example, here is the base commit tree.

Here you can see, there are two branches, master and development.

Merge

Merge is a very straight forward method to join branches.
When merge command is issued, git will compare current branch with the target branch and aggregate commits that are different from the diverge point, here is [3 4 5].
Then git will place the the aggregated commit [7] on top of the master. Also, conflicts is also placed in this commit.

As you can see, this method retains branch structure so reverting to old branch structure is possible.

Rebase

Different from Merge, Rebase does not retain branch structure.
The different commits will be replayed on top of the master branch, as if you commit them on master at the first place.
Commits will be replayed one by one. Conflict needs to be resolved on each commit.

As you can see, the branch structure is aligned as a straight line. So revertng directly to old branch structure is not easy.