Four file extensions, one family. All four carry the same thing — dated, signed bank transactions — but each accounting tool insists on its own dialect.
OFX — the open standard
Open Financial Exchange is the base format the others descend from. Xero, Sage, GnuCash, Wave, MYOB, and most non-Intuit software import OFX directly.
QBO — QuickBooks Web Connect
A .QBO file is OFX plus an Intuit bank-identifier tag, for QuickBooks Desktop and Online. Desktop validates that tag, which is why renaming an .OFX to .QBO doesn't work.
QFX — Quicken Web Connect
Quicken's flavor of the same idea. Structurally it's OFX with Intuit extensions, read by Quicken rather than QuickBooks.
QIF — the classic
Quicken Interchange Format predates OFX: plain text, one field per line. Old, but still widely accepted — Quicken and many older ledger tools import it, and it's a dependable fallback when a Web Connect import misbehaves.
Converting between them
The converter reads CSV, PDF, QBO, OFX, and QIF and writes all of QBO, QFX, OFX, QIF, IIF, and CSV — so whichever format you were handed, you can produce the one your software wants. CSV output is free; see the QBO viewer to peek inside a file first.