All smoking is done using the indirect method, but not all indirect grilling is smoking. One difference lies in the temperature. Indirect grilling is generally done at a moderate heat (most typically at 350°F, although you can indirect grill at as low as 300°F and as high as 400°F). Smoking is typically done between 225° and 275°F.
The other difference lies, of course, in the smoke itself. When indirect grilling, you can add soaked wood chips or chunks to the fire to obtain a smoke flavor, but you can also indirect grill without smoke (for example, Asian recipes grilled using the indirect method are often cooked without smoke). When smoking, you always add wood chips or chunks to the fire.