People use "bleed" and "flush" interchangeably, but they are different jobs with different goals. Knowing which you actually need saves confusion at the shop and helps you maintain your brakes correctly. Here is the difference.
Bleeding: removing air
Bleeding the brakes means removing trapped air from the hydraulic system to restore a firm pedal - see brake bleeding basics. You do it after a repair that let air in, after replacing a caliper or hose, or when the pedal goes spongy. Bleeding pushes out just enough fluid to clear the air; it does not necessarily replace all the fluid in the system.
Flushing: replacing all the fluid
A flush replaces essentially ALL the old fluid with fresh, cycling new fluid through until clean fluid runs out every bleeder - see brake fluid flush guide. The goal is to remove old, moisture-laden, contaminated fluid - see contaminated brake fluid. This is the scheduled maintenance item done every 2-3 years - see how often to change brake fluid.
The overlap
The two use the same basic process - opening bleeders and pushing fluid through - which is why the terms blur. The difference is scope and purpose: a bleed targets air with minimal fluid loss; a flush deliberately replaces the entire volume of fluid. A flush naturally bleeds out any air too.
Which do you need?
Spongy pedal after a repair? You need a bleed. Old dark fluid or hitting the time interval? You need a flush. Both keep the pedal firm and protect the system. Whichever you do, use fresh correct fluid and keep quality pads and hardware in good shape.