Every marijuana plant will look different when it is fully grown. The type of seed (AK 47, White Widow, etc) and gender play the major roles but loads of things can affects what the weed plant looks like when it is grown. Some plants will have a lot of crystals and that can be severely effects by how you grow it (indoor, outdoor, or hydroponic) but you can still get a general idea of what the average marijuana plant looks like.
Male marijuana plants will be a lot taller than female plants. They will not weigh as much though and the yield of marijuana leaves and bud is much less.
Female marijuana plants will be shorter but can weigh as much as double as a male plant. Female weed plants will grow marijuana leaves in clusters and these will be a lot easier to smoke then the buds you get from male plants.
The main difference is that female plants are a lot better and the end result is a bigger plant with a lot more product on it. If you want to smoke the pot then you are going to be wishing that all your seeds are females. Keep in mind that the visual presentation of the plant will vary a lot but the general size and height of the weed plant matches the small descriptions given in this article.