Update Item to Revision vs Revert to Revision
Update to revision will only update files of your workingcopy to your choosen revision. But you cannot continue to work on this revision, as SVN will complain that your workingcopy is out of date.
revert to this revision will undo all changes in your working copy which were made after the selected revision (in your example rev. 96,97,98,99,100) Your working copy is now in modified state.
The file content of both scenarions is same, however in first case you have an unmodified working copy and you cannot commit your changes(as your workingcopy is not pointing to HEAD rev 100) in second case you have a modified working copy pointing to head and you can continue to work and commit
To understand how the state of your working copy is different in both scenarios, you must understand the concept of the BASE revision:
BASE
The revision number of an item in a working copy. If the item has been locally modified, this refers to the way the item appears without those local modifications.
Your working copy contains a snapshot of each file (hidden in a .svn folder) in this BASE revision, meaning as it was when last retrieved from the repository. This explains why working copies take 2x the space and how it is possible that you can examine and even revert local modifications without a network connection.
Update item to Revision changes this base revision, making BASE out of date. When you try to commit local modifications, SVN will notice that your BASE does not match the repository HEAD. The commit will be refused until you do an update (and possibly a merge) to fix this.
Revert to revision does not change BASE. It is conceptually almost the same as manually editing the file to match an earlier revision.
The files in your working copy might look exactly the same after, but they are still very different actions -- the repository is in a completely different state, and you will have different options available to you after reverting than "updating" to an old revision.
Briefly, "update to" only affects your working copy, but "reverse merge and commit" will affect the repository.
If you "update" to an old revision, then the repository has not changed: in your example, the HEAD revision is still 100. You don't have to commit anything, since you are just messing around with your working copy. If you make modifications to your working copy and try to commit, you will be told that your working copy is out-of-date, and you will need to update before you can commit. If someone else working on the same repository performs an "update", or if you check out a second working copy, it will be r100.
However, if you "reverse merge" to an old revision, then your working copy is still based on the HEAD (assuming you are up-to-date) -- but you are creating a new revision to supersede the unwanted changes. You have to commit these changes, since you are changing the repository. Once done, any updates or new working copies based on the HEAD will show r101, with the contents you just committed.