Navigation grid
The world is divided into a grid of free and blocked cells that is searched with A*.
The simplest representation for pathfinding: lay a uniform grid over the level, mark each cell walkable or blocked, and let A* find the route between cells. Cheap to build and update, which makes it standard in real-time strategy, tower defence and tile-based games. Allowing diagonals gives eight neighbours per cell.
The cost is blocky paths that hug the grid and need smoothing afterwards, plus a lot of nodes on open ground. There a navmesh is preferred, covering the same area with far fewer polygons. Flow fields are an alternative when hundreds of units head for one shared goal.