Pools are groups of posts with a common theme. Posts in pools can be ordered; this makes pools ideal for posts that are part of a series (e.g., comics).
To create a new pool, click the "new" button at the top of the pools page or click here.
Pools have strict moderation since they are shown publicly and prominently at the top of a post's page. Adding posts to pools they don't belong in or creating pools with subjective themes (e.g., "hot pics") will not be tolerated. If you want to group posts subjectively, see the help page on sets.
The interface for adding and removing pools resembles the interface for favorites. You can click on Add to Pool from the post's page. You'll be shown a form where you can select a pool to add the post to.
If you're importing several posts into a pool, this process can become tedious. You can instead click on the Import link at the top of the pool's page. This allows you to execute a post search using any tag combination you would normally use. Remove any posts that are irrelevant to the pool, then finish the import process.
To remove a post from a pool, go to the pool's page and select the Delete Mode checkbox. Then click on the posts you want to delete. This works similarly to how posts are deleted from favorites.