git remote add just creates an entry in your git config that specifies a name for a particular URL. You must have an existing git repo to use this.

git clone creates a new git repository by copying an existing one located at the URI you specify.

These are functionally similar (try it!):

 # git clone REMOTEURL foo
and:

 # mkdir foo
 # cd foo
 # git init
 # git remote add origin REMOTEURL
 # git pull origin master
Now there are minor differences, but fundamentally you probably won‘t notice them. As an exercise left to the reader, compare the .git/config‘s from each directory.