When you save artwork in a bitmap format—such as JPEG, GIF, or PNG—Illustrator rasterizes the artwork at 72 pixels per inch. You can preview how objects will appear when rasterized by choosing View > Pixel Preview. This is especially helpful when you want to control the precise placement, size, and anti-aliasing of objects in a rasterized graphic.

To understand how Illustrator divides objects into pixels, open a file that contains vector objects, choose View > Pixel Preview, and magnify the artwork so that you can see its individual pixels. The placement of pixels is determined by an invisible grid that divides the artboard into 1‑point (1/72 inch) increments. If you move, add, or transform an object, you’ll notice that the object snaps to the pixel grid. As a result, any anti-aliasing along the “snapped” edges of the object (usually the left and top edges) disappears. Now deselect the View > Snap To Pixel command, and move the object. This time, you’ll be able to position the object between the grid lines. Notice how this affects the anti-aliasing of the object. As you can see, making very minor adjustments can affect how the object is rasterized.

Pixel Preview off (top) compared to on (bottom)