Archives are collections of published or (more frequently) unpublished records that have been selected for longer term preservation. As a rule, records in an archive are no longer used for their original purposes by their creators.
The reason that much of this site concentrates on archival holdings held by university libraries is that these archives are often unique and therefore unavailable elsewhere. Normal library holdings are typically published, and therefore, in theory, should be available in any library. Unpublished archival holdings are almost invariably unique, and therefore to view them you have to go to the institution (or the institution's web site if you're lucky) that holds them.
They may be held for reference 'just in case', or because the organisation is legally bound to keep the records for a certain length of time (this is often the case with legal and financial documents).
Their preservation may be intended to be permanent or may be due to be reviewed at some specified point, for example after seven years or once the shelves upon which they're stored become full.
Rules regarding access, reproduction etc might also be reviewed at specified points, for example as with the old 'thirty year closure rule' for some government documents.
Because archival material is always treated as unique, you won't be allowed to borrow items in the same way as you might borrow library books.
Reference works in libraries are generally organised according to their subject, as identified by the librarian. Archives are different in that they tend to be organised according to where and when the archive acquired them from (their 'provenance'). When an archive receives a batch of records, the archivists don't split the material up & assign it to different parts of their existing collection, but try to maintain the records' 'original order'. This means that individual archival files often contain very different sorts of historical information.