How the surface gets there
Every batch of ready mix has stone in it, from pea size up to something closer to a marble, and normally that stone stays buried under a skin of cement paste that finishing tools work smooth. Exposed aggregate flips that. A surface retarder gets applied to the top of the slab either as the concrete is placed or right after it is struck off, and that retarder keeps the very top layer of paste soft while the concrete underneath cures normally.
Once the slab has firmed up enough to walk on carefully but before that top layer hardens, the crew washes and lightly brushes the surface, taking the softened paste off and leaving the stone standing just proud of it. Timing is the whole job here. Wash too early and stone comes loose with the paste. Wash too late and the retarder has stopped working, and the paste will not come off without a grinder.
What it looks and feels like once it is done
The result is a surface with real texture, stone underfoot rather than a smooth or lightly brushed skin. That texture hides dirt and everyday wear better than a plain broom finish does, and it stays grippy wet without needing a heavy pattern the way a stamped surface does. It is a common pick on pool decks and walkways for exactly that reason, grip without the sealer upkeep that comes with color.
Because the finish is the stone the concrete already contains, exposed aggregate does not carry a separate color decision the way stamped work does. The look comes from the aggregate in the mix, so the range of tones a job can land in is set by what is naturally in the ready mix rather than by pigment choices made at the estimate.
Where it sits next to broom and stamped finishes
Exposed aggregate lands between the other two finishes rather than replacing either one. A broom finish is the simplest and grips the least aggressively. Stamped work carries the most upfront and ongoing effort, since color has to be sealed and resealed. Exposed aggregate wears more like a broom finish, plain and low maintenance, while giving a texture that reads as more deliberate than a broomed surface. The full side by side on cost and upkeep between all three lives on our patios page, since that comparison belongs there and not repeated here.