Ventral means toward the belly and dorsal means toward the back.

Example: The kneecap is found on the ventral side of the leg while the shoulder blades are placed on the dorsal side of the body.

Note that ventral and dorsal can in most cases be exchanged with anterior and posterior; however, in other cases the terms are not interchangeable. Such a case is the anatomy of a four-legged animal or a fish. For a fish, anterior describes a structure located toward the front, in this case, toward the head, whereas ventral describes a structure located toward the belly. Same for the other two terms, posterior describes a structure located toward the backside or buttocks, in this case, toward the tail, whereas dorsal would reference the top of the fish where the dorsal fin is found. See illustration below:

Image 1: Ventral, dorsal, anterior and posterior on a fish