Object snaps (Osnaps) are one of the handiest tools in AutoCAD. Without them, you would have a lot of trouble doing even the simplest dimension accurately. Before discussing how they are used, first think about what they are. Their name, "object snaps" means that they snap to objects, or more precisely, they snap to specific parts of an object. A line for example has 3 points that you can snap to: a midpoint and two endpoints. A circle has 5 points: a center and 4 quadrants. To use Osnaps effectively, you must know what points an object has that you are able to snap to.
In this AutoCAD tutorial we'll cover: "Snap", (not to be confused with "object snap") - acts like a magnet to force the cursor to set increments of the screen. The "grid" is a matrix of dots on the screen that can help you visualize things.