Undoing a revision undoes one edit. It reverts the edit window to the previous version, adds an automatic edit summary (which can still be changed) and then you have to save it.
A rollback is an anti-vandalism/anti-spam/generic cleanup tool only given to admins and rollbacks (and content mods?). It allowed you to, with one click, roll back all edits made by the last user. So if they made five edits, it rolls them back all at once. It goes automatic; does not offer the opportunuty to add an explanation. It's useful if you have to undo a lot of edits by a single editor, for example.
Both can be done from Recent Changes, History, Diffs and Contributions.
Just to clarify, when using rollback, the edits need to be consecutive. If, for example, user1 made 5 edits, user2 made an edit, and then user1 made another 5 edits, only the 5 most-recent edits by user1 get undone, not all 10.
Sometimes edits can be individually undone if they are in different sections of the page. But not if in the same section, which gets you a yellow banner message of "this edit cannot be undone".
For example, if edit3 is in the lede (first) paragraph, and edit5 is in "Trivia", and all the other edits by that user all over the place, then you have a good chance of being able to undo edits 3 and 5.
As Love Robin pointed out, undoing non-consecutive edits may or may not be possible depending on what the edits where and what the edits between them where.
However, if the edits are consecutive there is a third option; manipulate the editing URL directly to contain the relevant revision ids. Let us use my previous example situation. Each edit is assigned a revision id (revid). For the sake of simplicity, let us assume the revid is the edit number counting from one. So, the page as created would be revid1, after the first edit revid2, after the second edit revid3, and so on.
If you use the undo feature, you will end-up in the "special" undo editor mode (I am not sure what else to call it). If you look at the URL in your browsers address bar, you will notice this at the end of it: "&undoafter=revid10&undo=revid11". If you use the rollback feature, it would be "&undoafter=revid6&undo=revid11". The pattern here is that "undoafter" is given the revid of the edit you want to keep and "undo" is given the revid of the current version. Everything after the "undoafter" revid is, well, undone.
Now let us consider how to use this to undo consecutive edits in a customized manner. If you wanted to undo only the 2 last edits by user1, not all 5. You could use "undoafter=revid9&undo=revid11". Now let us say you want to rever the page all the way back to user1's 4th edit. The you could use "undoafter=revid4&undo=revid11". IN the latter case, you are actually able to undo consecutive edits that involve multiple users.
In the example, we used a simple system for assigning revision ids and assumed that revisions to other pages weren't made between revisions to the page we were interested in. This may not be the case on an actual wiki. So, if you really wanted to use this method, how do you get the revids? Well, there are several other links on the history page that use the revid; So we can get it from those. The easiest one to use is the timestamp. If you click on the timestamp, you will be taken to view that particular revision. If you look at the URL, you will notice "?oldid=" near the end of the URL. The number on the other side of the equals sign is the revision number.
Note that using this method will not produce an automatic edit comment.