Different strategies to merge Git branches?

In bitbucket merging a pull request offers different strategies (Merge strategy), which are:

# Merge commit 
git merge --no-ff 

# Squash 
git merge --squash 

# Fast forward 
git merge --f-f-only

I've only used the first one, which comes by default.

What are the consequences of using each of them? In what situations is each appropriate?