Difference Between Male and Female Plants

Differences

The male will tend to grow taller faster and be less bushy, but not always.

Preflowers, as opposed to full blown flowers, generally but not always appear after the fourth -fifth week of vegetative growth from seed. Check carefully above the fourth node. Preflowers are very small and and near almost impossible to differentiate without magnification.

Preflowers are often pear shaped and produces a pair of pistils but it's common for only one pistil or none to display at early age.

Cannabis plants do not all show sex at the same age. It depends on the strain mainly. I have had some plants show sex at 5 weeks. Other strains, like the Hindu Kush, did not show sex until much later, like around 2 - 3 weeks of flowering, when they were 8 weeks old!

In the early stages preflowers might often not show pistils until well advanced so do not start pulling plants until you are 100% sure.

Pistillate preflowers are located at the node between the stipule and emerging branches.
Some times female preflowers never produce pistils. A female preflower without pistils is difficult to distinguish from a male preflower so don't be fooled into thinking it's a male or a hermaphodite .

Female Hairs

Female Pistillate

Once your seeds begin to grow into a plant you need to sex the plant. You need to sex the plant because only the female plants produce the buds you want to smoke. If you are growing under lights you can do this by putting the plant on a light cycle of 12 hours on 12 hours off. After a week or two you will begin to see small white hairs emerging from the sacs at the base of the branches. If you see hairs coming out of the sacs you have a female. If the sacks develop but no hairs emerge you likely have a male. Unless you are going to seed out some of your plants you will want to get rid of the males as soon as possible

Male Preflower

Male Flowers

Female Pistils - notice the sac and the feathery frond-like structure. Usually after five days to two weeks you will begin to see a pair of tubular nodes. Hopefully they will have white hairs protruding from them as this denotes a female plant. These little hairs are known as pistils and they are the first sign your plant is flowering.
Female Cannabis – The female cannabis plant, unlike the male, is grown to produce marijuana. Premium marijuana is produced in seedless form by eliminating all pollen sources from the growing environment. Seed production reduces the value of recreational marijuana dramatically by lowering the yield and potency of the flowering and fruiting tops.
The female flowers appear as two long white, yellow, or pink pistils protruding from the fold of a very thin membranous calyx. The calyx is covered with resin exuding glandular trichomes (hairs). Pistillate flowers are borne in pairs at the nodes one on each side of the petiole behind the stipule of bracts which conceal the flowers. The females are recognized by the enlargement of a symmetrical tubular calyx (floral sheath). They are easier to recognize at a young age than male primordia. The first female calyxes tend to lack paired pistils (pollen-catching appendages) though initial male flowers often mature and shed viable pollen. In some individuals, especially hybrids, small non-flowering limbs will form at the nodes and are often confused with male primordia. The female plants tend to be shorter and have more branches than the male.

More? See http://www.erowid.org/plants/cannabis/cannabis_cultivation3.shtml

Or http://www.weedfarmer.com/gallery/slideshow.php?set_albumName=male-female

There are two obvious types of hermaphrodite, like above with male sacs among the heads, or with one branch of a female plant being entirely male.