The full list of accounts every transaction in the business posts to.

Bookkeepers and accountants setting up or maintaining the books
Chart of Accounts is the backbone of the books - every transaction, invoice, and bill ultimately posts to an account defined here. Set it up once and every report downstream (Balance Sheet, Income Statement, Trial Balance) is generated from it.
See how Chart of Accounts fits into a plan on pricing, or how Count My Bucks compares to QuickBooks.