Masonry paint is the stuff you put on the outside of a building — brick, render, cement, concrete, pebbledash, stone, breeze block. It's built to cope with everything British weather throws at a wall: driving rain, frost, UV, and the constant freeze-thaw cycle that cracks lesser coatings. The key difference from interior emulsion is flexibility and breathability. Exterior walls move and they need to let moisture vapour out, so masonry paint is formulated to flex without cracking and to breathe rather than trap damp behind a film.
You need it whenever you're painting any exterior wall surface. Don't be tempted to use leftover interior emulsion outside — it'll blister, flake and look dreadful within a season.
There are broadly two types. Smooth masonry paint gives a clean, even finish and is the usual choice for render and smooth surfaces. Textured masonry paint has fine aggregate in it to bridge hairline cracks and add a bit of grip — handy on rougher or slightly tired surfaces.
For topcoats, Little Greene Masonry Paint is genuinely excellent — properly breathable, low-VOC, and it comes in their full colour card so you're not stuck with builder's magnolia. Dulux Weathershield is the reliable workhorse with a long guarantee, and Sandtex is the go-to for textured exterior finishes and is hard to beat for value on a big elevation.
The "but what about" question I always get: do I need a primer? On bare, chalky or previously-unpainted render, yes — use a stabilising primer (Sandtex and Dulux both do one) to bind the surface and stop the topcoat soaking in unevenly. On sound, previously-painted walls you can usually go straight on after a wash down.
Practical advice: paint in dry, settled weather — ideally above 8°C and not in baking sun, which flashes the surface and ruins the finish. Two coats, always. And give the walls a proper scrub and a fungicidal wash first if there's any green growth, or it'll bleed straight through your fresh paint.