If you go to Add-ons -> Show Available Add-ons, there's a brief description of what each one does. So many, that do so much...which ones will be of the most use to you at any given time will vary depending on what you're trying to achieve. But for the most part they're fairly self-explanatory, and can best be leveraged in campaigns (e.g. scrape, check alive, check PR of alive, et cetera before blasting as one example).