Use Quickbooks compatible formats (QBO or IIF)

QuickBooks accounting software is offered as QuickBooks Desktop (QBD) or QuickBooks Online:

Quickbooks Desktop: the QBO format until it becomes three years old and IIF format
Quickbooks Online: the OFX/QBO/QFX format and the CSV format

Use the ProperConvert app to convert your file to a format compatible with Quickbooks.

Import into QuickBooks through the QBO format

The QBO format is a QuickBooks Web Connect format used to import bank transactions into QuickBooks (under Bank Feeds). ".QBO" does not mean "QuickBooks Online".

Banks and credit card companies provide QBO files to download transactions. QBO files have account details like account id, bank id, currency, ending balance, and date. QBO file have the following transaction details:

date
amount
payee
description
check number
unique transaction ID

Quickbooks imports the .QBO files under Bank Feeds. As banks do not have specific accounting details for their clients, the .QBO files do not have Categories or expense/income account details. Once transactions are imported under the Bank Feeds, Quickbooks does matching to vendor records and expense/income accounts.

Import into Quickbooks Online

QuickBooks Online (except Self Employed) imports QBO files (OFX, QFX as well)
The QBO (and OFX and QFX) format is the best format for Quickbooks Online as it imports the most bank transaction details (date, amount, payee, memo, check number, unique transaction id to exclude duplicates).
Quickbooks Online (including Quickbooks Self-Employed) imports CSV files (date, amount, description only).

Import into QuickBooks Desktop through the IIF format

IIF format is the format to use for all QuickBooks versions. It allows supplying more details about transactions like expense/income account or class. Unlike the QBO format, which requires "matching" under the Bank Feeds after QBO import, the IIF format requires vendor names, account names to be exact to what you have in Quickbooks otherwise Quickbooks will create new vendor and account records during IIF import.

Quickbooks Online does not import IIF files.

The IIF format is considered a "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 a backup (as with any other import) for the QuickBooks company file before any IIF import. Putting aside the scary part, this is a great format widely used and you can import data quickly. If you just figuring out QuickBooks, start with the QBO format (see above).

Intuit developed the IIF format for its QuickBooks products and many software packages support export in this format for transactions, sale receipts, bills, invoices, and other data lists. As IIF is considered a "system-level" format where data are imported directly into Quickbooks data file.

For transactions, IIF format supplies the following details:

date
amount
vendor or customer[:job]
description
main account
expense/income account
class

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.

Use the ProperConvert to convert your transaction files to the IIF format.

What are the main differences between QBO and IIF import into QuickBooks?

QBO import requires an Internet connection with QuickBooks allowed to access the Internet, IIF import is completely offline
QBO import allows to apply renaming rules to assign payee or vendor even if it is different on supplied QBO file from the record in QuickBooks, and assign the expense account during import. IIF must have expense accounts supplied as you have them in QuickBooks. Bank2IIF allows to do that through the category column or provide a default expense account.
Imported QBO transactions go first into the Online Banking Center, where you can review, delete, match and finally include them into the register. QuickBooks provides two Online Banking Center modes, completely be their user interface and actually how transactions are imported. IIF import is done directly into the QuickBooks transaction register.
QBO import requires QuickBooks to be no older than three years. IIF keeps importing without asking you to upgrade.
QuickBooks Online imports QBO files only, QuickBooks Online does not import IIF files.

Learn More

QBO - What is QBO (web connect) format
QFX - What is QFX file/format and how to open it?
OFX - What Is OFX File Format and How to Open it?
IIF - What Is IIF File Format and How to Open it?
QuickBooks Online - Import a Transactions file into Quickbooks Online
QuickBooks Desktop - How to Import Bank Transactions into QuickBooks Desktop