A cash machine is the everyday British name for an automated teller machine (ATM), the self-service terminal that lets a customer take out cash, check a balance and do simple banking without visiting a cashier. In the UK it is also called a cashpoint or, informally, a "hole in the wall". The full explanation of how these machines work, who runs them and what they charge is on the ATM page.
A customer uses a debit card and PIN to make a withdrawal from their bank account. Some machines also accept deposits of notes and cheques, and in many countries machines run by independent operators charge a fee that a bank's own machines do not.