Note also that while tags aren't case-sensitive for searching, they are registered in the database with the capitalization of their very first use, which is applied to every subsequent invocation. If you're pedantic like me and goof on a character name, you can ask @BadKarma to adjust it manually.
They're also incredibly flexible for casual searches, so while there doesn't seem to be a hard limit on total tags, you can opt to economize around fewer but more specific labels ("old man" v. "old"/"man" &c.).
Regarding Serials, while the upload form only lets you submit to one at a time, you can add a work to multiple serials in gallery management the same as Portfolios, so you can run separate tracks for the "main" story, the asides, and everything together. There used to be a link in a work's description that switched to the Serial UI when accessed from the main gallery, but I don't see it now – @BadKarma possible bug?
(That could only handle one at a time: it linked to whichever serial the work was last saved to.)
Welcome to the site!
Adding tags to your uploads is pretty simple. On the main upload page as well as on the edit page, just under the description box, you can put a comma-separated list of tags.
You can create as many serials folders as you need. Once you have a serials folder set up, you can view everything in that folder in a more traditional web comic style.
Hope that helps, and that you enjoy the site!
-- BK