Good marijuana gives off a clean, sharp, pungent smell.
Some marijuana varieties have a citral smell similar to the scent of a lemon tree, other marijuana variaties have a spicy scent or even an ammonia/chemical-like smell, this is perfectly normal. If you can smell something resembling freshly mown grass, the marijuana is still too "green" (wet) and was not given enough time to dry, or "cured" properly.
For "brown" marijuana like Thai, Jamaican, Columbian, etc., you should be looking for tight, well dried chunks, or pressed layers. There should be few seeds and stems, and no contaminates like dirt.
For "green" marijuana like Skunk, Widow, B52, etc., generally what you're after are "Nugget-like", tight buds with the larger leaves removed. However, with "Afghani Kush" and other marijuana hybrids, it is perfectly natural for there to be medium sized leaves wrapped around the buds, protecting their potent white THC crystals. (THC / tetrahydra-cannibinol is the main active ingredient in marijuana and cannabis. It gets you stoned! It is concentrated in the buds and in the powdery white crystals that cover most buds and some leaves.) Finally, good "greens" are usually "sensemilla", meaning without seeds!
Sativa strains generally have longer and looser buds and leaves while growing. These strains like Thai, and Columbian are usually packed or compressed into bricks for shipment. As a result, they tend to be hard, flakey, and compact.
Skunk varieties will crumble easily, often feeling powdery and dry. Hybrids such as "Kush" and "Northern Lights" will always feel sticky and moist no matter how dry, and are best cut with scissors.