BKPF is the document header table. It stores all the fields common to all of the document line items such as posting date, currency key, document number etc.
BSEG is the associated line item table to BKPF. Specific line item information such as posting key, GL account, document amount are stored.
GLT0 is the summarized account balances table showing account balances by period, by account, by fiscal year.