What's the difference between brake discs and brake pads?

Brake pads are the friction material that clamps onto the brake discs to slow the car down. Brake discs are the metal rotors themselves, mounted on each wheel, that the pads press against.

The two wear together, but at different rates: pads wear faster and are a routine, relatively inexpensive replacement item. Discs wear more slowly, but not indefinitely - they can also develop grooves, rust, or warp over time, especially if left in place through several sets of pads.

In practice, a brake job sometimes means pads only, and sometimes means pads and discs together, depending on how worn the discs are underneath. It's a reasonable question to ask a garage which was actually needed and why, rather than assuming it's always both.