For every accounting document there is a corresponding entry in the General Ledger (this is the same with almost every ERP and Accounting System in use today).

Documents:

• account.invoice - stores Invoices and Refund Invoices (credit notes) for Customers and Suppliers

• account.invoice.line - stores the lines from an Invoice

• account.voucher - stores Payments

• account.voucher.line - stores the lines from a Payment.

Entries:

• account.move - stores Journal Entries
• account.move.line - stores the lines from a Journal Entry (Journal Items)

These are ALL important.

A business document is a historical representation of a transaction and the general ledger is the central repository (based on double entry bookkeeping) recording those transactions so the impact on your business can be measured.

I suggest reviewing some background Accounting information, like http://www.principlesofaccounting.com/