Invoicing apps typically let you create, email and track invoices and generate historical reports. They usually don't include General Ledger (GL) and Accounts Payable (AP) capabiliites--which accounting software does. They also don't provide inventory mgt., which several entry level accounting software packages do.

If you are a solo business owner, especially in a services business, an invoicing app like Freshbooks will probably be all you need. It also tracks expenses, clients and contractor staff.