QuickBooks accounting software is offered as QuickBooks Desktop (QBD) or QuickBooks Online. Both variants import QBO format as a common format, and each of them import their set of formats. See below options and Desktop and Online versions.
QuickBooks Desktop imports QBO, IIF, QBJ files. If your QuickBooks version is older than three years, use the IIF format. If your QuickBooks 2013 or later, you may also use QBJ format. QuickBooks Online imports QBO, QFX, OFX, CSV (limited) files.
Your QuickBooks version must not be older than three years to import QBO files. QBO (Web Connect) files are imported under Bank Feeds in QuickBooks where they are matched to vendor records and assigned expense/income accounts. QBO files do not carry expense/income accounts (categories) as the categorization is done after import in QuickBooks.
The IIF format is considered "low level" or "system" format and QuickBooks imports it right into its database, so you have to be precise with the data you provide. Typos and mismatches will result in new vendor records and accounts created. Always do backup (as with any other import) for QuickBooks company file before any IIF import. Putting aside the scary part, this is great format widely used and you can import data quickly. IF you just figuring out QuickBooks, start with the QBO format (see above).
CSV files must have exact vendor names and category (expense/income) account names matching account names in QuickBooks. All QuickBooks versions are supported as long as they import IIF files (except QuickBooks Online). For QuickBooks Enterprise the user must have exclusive access to import IIF files.
QBJ is general journal entry format. This converter is different from other converters. You have to use special Excel template and provide all account details.
QuickBooks Online imports QBO, QFX, OFX, CSV files. Categorization is done after import in QuickBooks Online. CSV files have a limited support and have to follow expected layout. Use converters below to create importable CSV/QBO files from your Excel/PDF files.