They're different things. An apron just covers the front of your body. It usually has a strap behind your neck and strings round your waist which you tie behind you. A smock is a complete garment, like a loose-fitting dress. You put a smock on over your head, and it covers most of your body. Or just look on Google Image.

The terms are similar, but not interchangeable. A smock is a loose shirt or dress like garment, whereas an apron is a long protective garment worn over ones clothes when performing messy tasks like cooking or working as a blacksmith. Smocks are generally considered women's clothing, but aprons are worn by both men and women, though a traditional woman's apron (like what a 1950's housewife would wear) has lace and frills and is, therefore considered a feminine garment.