When a client wants to import data, such as past transactions into QuickBooks, the initial assumption is that Intuit has an easy solution. The reality is that QuickBooks does not always play well with others (like PayPal), but there are a few decent options for importing journal entries, transactions and any kind of list (like items, vendors, customers etc.).
What makes this process confusing to most is that the file formats are not common to the typical computer user, so I organized them all there:
1) Web Connect Files (.QBO): This is a file that allows you to connect with your online bank account and pull the latest transactions from your bank account. For the desktop version of QuickBooks, you can set this up by going to Banking>Bank Feeds>Set Up Bank Feed for an Account. This option is only available if your bank participates with Intuit and you can only import transactions from the bank account. You cannot use a .QBO file to import non-bank data (such as if you are logging transactions elsewhere or with another type of software and wish to bring them into QuickBooks). If your bank does not support the .QBO format, but you are able to download your bank data into a .CSV file, then try a CSV to QBO Converter.
2) Intuit Interchange Format (.IIF): This is how all of your lists can be moved from one QuickBooks (or Quicken) file to another. You can export and import all types of lists including your chart of accounts, customer list, vendor list, employee list, class list and more. This is also the most versatile format for moving data into QuickBooks because you can import .IIF files for all types of transactions: checks, cash sales to customers, account transfer of funds, and perhaps most importantly, general journal entries. This means that you could log accounting transactions in another software and bring them into QuickBooks through the .IIF format. Since this has become more popular, a few third party applications are popping up that allow you to convert Excel files to .IIF files, but you can also easily create yourself by following the directions from Intuit’s website (make sure your headers are correct).
3) Excel Files (.XLS): For certain lists (Customers, Vendors, Items and Inventory Items), you can import information from an Excel file. This is the easiest way to import data and many people wish it were this easy to link QuickBooks with other kinds of data.
4) QuickBooks Journal Entries (.QBJ): This is mostly helpful for accountants and bookkeepers, but you can create a journal entry in QuickBooks and then send that entry to another user who can then import it into their file. This differs from creating a journal entry in an .IIF file, but you get a similar result.