a grain
Lay floor tiles across a big room but let several people start tiling from different corners, each keeping their own tiles neatly lined up. Where two people's patches meet, the rows do not line up and you get a visible seam. A metal solidifies the same way: crystals start growing from many separate points in the melt, each a perfectly ordered patch but tilted at its own random angle, and they grow until they run into each other. Each of those ordered patches is a grain — a single-crystal region within a larger polycrystalline solid.
Inside one grain the atoms sit on a single, unbroken crystal lattice with one consistent orientation — it is a little single crystal. The surface where one grain meets its differently oriented neighbour is a grain boundary, a thin zone of mismatched atoms (the seam between the tile patches). A typical engineering metal is a polycrystal of enormous numbers of grains: a 1 cm cube of steel with 30-micrometre grains contains on the order of 10^10 grains. Grains are usually invisible to the eye but appear clearly under a microscope after polishing and etching, because the boundaries etch faster and show as dark lines.
Grains are the basic unit of microstructure, and their size, shape and orientation govern much of a material's behaviour. Smaller grains generally make a metal stronger, because there is more grain-boundary area to block the motion of dislocations (the Hall-Petch effect). One honest caution: a grain is not the same as a crystal unit cell. The unit cell is the tiny repeating box of the lattice (fractions of a nanometre); a grain is a macroscopic patch, often tens of micrometres, containing billions of unit cells all sharing one orientation.
Etch a brass sample and photograph it: you see irregular polygons a few tens of micrometres across, meeting along dark lines at roughly 120-degree triple junctions. Each polygon is one grain — a single crystal of brass — and the whole picture is a slice through the polycrystal.
Grains show as polygonal patches; the dark network between them is the grain boundaries, where orientations mismatch.
A grain has one crystal orientation, but its shape is set by how it grew and bumped into neighbours, not by the crystal's symmetry — so grains look like irregular blobs, not like faceted gemstones, even though each is internally a perfect crystal.