What is in the general ledger?
Everything. Sales invoices, purchase bills, bank payments and receipts, payroll, VAT, journals — each one is posted as a set of debits and credits to named accounts, and the ledger is the sum of all those postings. Look up any account and you see every entry that touched it, in date order, with a running balance.
The accounts are the ones listed in the chart of accounts and identified by nominal codes. Sales, materials, rent, bank, debtors, creditors, VAT, capital: each has its own page in the ledger. The trial balance is simply the closing balance of every account listed on one sheet, and the profit and loss and balance sheet are those balances grouped and subtotalled.
In older systems the general ledger sat alongside separate sales and purchase ledgers, which held the detail of who owed what. Modern software keeps the detail and the totals together, but the term has stuck, and 'the ledger' still means the single place where the books actually live.