I didn't want to lose some information after a git pull, so I did a git fetch before. Where can I read the new modifications after a git fetch? I went to the FETCH_HEAD file, but there was nothing more than a big number.
git fetch origin by default fetches everything from the remote named "origin" and updates (or creates) the so-called "remote-tracking branches" for that remote. Say, for the remote named "origin" which contain branches named "master" and "feature", running git fetch remote will result in the remote-tracking branches named "origin/master" and "origin/feature" being updated (or created, if they're not exist). You could see them in the output of git branch -a (notice "-a").
Now, the usual Git setup is that (some of) your local branches follow certain remote branches (usually same-named). That is, your local "master" branch follows "origin/master" etc.
So, after you fetched, to see what remote "master" has compared to your local "master", you ask Git to show you exactly this:
git log origin/master ^master
which means «all commits reachable from "origin/master" which do not include commits reachable from "master"» or, alternatively
git log master..origin/master
which has the same meaning. See the "gitrevisions" manual page for more info, especially the "Specifying ranges" part. Also see the examples in the git-log manual page
You're free to customize the output of git log as you see fit as it supports a whole lot of options affecting it.
Note that your local branch might also have commits which the matching remote branch does not contain (yet). To get an overview of them you have to reverse the revisions passed to git log for (hopefully) obvious reasons.
As usual, it's essential to educate yourself to understand the underlying concepts before starting to use a tool. Please do.