A call for abstracts is the announcement inviting researchers and practitioners to submit proposals for review, with a stated topic scope and closing date.
Scope, eligible formats, word limit, review criteria, the deadline, the notification date, and whether presenters must register and pay. That last one causes more friction than anything else when it is left unsaid, because a presenter who assumed a free pass is an awkward conversation three weeks out.
Largely interchangeable. Call for papers implies a full written paper will follow and is standard in academic and scientific conferences. Call for abstracts stops at the summary. Use whichever your audience uses, and say explicitly which one you mean if there is any doubt.
Most submissions arrive in the final forty-eight hours regardless of how long the window is. A long window does not produce more submissions, it produces the same submissions later. Set a date, hold it, and expect a surge at the end.
Last year's submitters, last year's attendees, the relevant societies and association mailing lists, and the people you personally want in the program. Open calls rarely fill a track on their own. The strongest sessions usually come from direct invitations alongside the open call. See abstract management software.