A snap grid and a visible grid are two main types of grids in PCB. The visible grid is an optional on-screen grid of solid or dashed lines, or dots. You can have a snap grid or visible grid set to different units (metric or imperial). Many designers prefer a 100 thou visible grid and rarely vary from that.
An electrical grid is also used in some programs. This grid is not visible but it makes your cursor snap onto the center of electrical objects like tracks and pads. This is extremely useful for manual routing, editing and moving objects.
One last type of grid is called component grid. It works the same as the snap grid but only for component movement. Component grid allows you to align components up to a different grid.