Transactions, just like double entry accounting transactions, are groups of balance updates that are ordered within a block. The Transactions endpoint returns a list of accounts, which have the following fields:
Starting sequence number (global ordering of the first update in this transaction relative to all other updates recorded on the ledger up until that point) for the balance updates occurring in this transaction
max_chain sequence number
Last sequence number (global ordering of the last update in this transaction relative to all other updates recorded on the ledger up until that point) for the balance updates occurring in this transaction