After posting about archives, I'm realizing that I can't remember ever using an archive.
The Internet Archive is a library, according to its homepage and Wikipedia. Is AO3 a true archive, or also more of a library? I've read a couple fanfics there, like the Old English Star Trek fanfic: https://archiveofourown.org/works/42430848
Anyway if I want to better understand what an archive is, what are some archives I can and should access in the Philly area or online? What do you use an archive for?