Marijuana plants are either male or female . The male Marijuana plants produce pollen which pollinates the flowers  of  the  female  Marijuana plant, which once pollenized, produce seeds . If the female Marijuana plant  isn't  pollenized  (if  there  are  no  male  Mariuana plants  nearby  producing pollen), the flower/buds continue to develop and produce THC. Female Marijuana plants  which  are  not  pollenized  are  referred  to  as sinsemilla (without seeds). Usually 30-50% of the Marijuana plants are male.

What's the Difference you ask?

Males are often, but not always, tall with stout stems ,  sporadic  branching  and  few  leaves.  Males  are  usually  harvested except those used for breeding,  after  their  sex  has  been  determined,  but  before  the  pollen  is  shed.  When  harvesting, especially if close to females, cut the Marijuana plant off at the base, taking  care  to  shake  the  male  as  little  as  possible.  This  helps prevent any accidental pollination by an unnoticed, open male flower.

When a male enters the stage of flower development, the tips  of  the  branches  where  a  bud  would  develop  will  start  to grow what looks like a little bud (little balls) but it will have no white hairs coming  out  of  it.  Females  will  have  no  balls  and will have small white hairs. Read More about Male marijuana plants.

Male marijuana plant 

Cannabis in temperate climates begin to show his sexual identity by the end of July (end of January in the southern hemisphere) in different dates according to the varieties, Marijuana being the resinous flower of female cannabis plants intended for seed production, in absence of pollen buds turns out pure sensimilla weed and is gentle and sweet to smoke.

It is very important to get rid of male plants on time, as they are unwanted pollen carriers. By the early flowering stage male cannabis, if compared to female's, shows quite a different structure but the characteristic excrescencies would be the sex indicator this are called primordia and will emerge by the side of the third or fourth internodes in the main stem.

Female cannabis are completely revealed when the characteristic "V" shaped pistils become visible, all this to a close observation. Outdoors males will uncover themselves approximately three weeks before the females, indoors sexing of both males and females happens within a week to ten days according to the variety.

Female marijuana plant 

We've heard of urban legends about environmental conditions, age of seeds, added chemicals and even lunar stages having an influence on sexual differentiation of Cannabis; you might take note of those suggestions as personal communications, but a good handbook or an internet surf works the best if you lack in experience when sexing.

What is a Hermaphrodite plant?
 
An hermaphrodite, or hermie, is a Marijuana plant of one sex that develops the sexual organs of the other sex. Most commonly, a flowering female  Marijuana plant  will  develop  staminate flowers, though the reverse is also true. Primarily male hermaphrodites are  not  as well  recognized  only  because  few   growers  let  their  males  reach  a  point  of flowering where the pistillate would be expressed.

Hermaphrodites are generally viewed with disfavor. First, they  will  release  pollen and ruin a sinsemelia crop, pollinating themselves and all of the other females in the room. Second, the resulting seeds are worthless, because hermaphrodite  parents tend to pass on the tendency to their offspring.

Please note that occassionally specious staminate  flowers  will  appear  in  the  last  days  of  flowering  of  a  female  Marijuana plant. These do not drop pollen and their appearance is not considered evidence of deleterious hermaphroditism.

Hermaphrodite marijuana plant 

Here's an image of a hermaphrodite, specifically a female Marijuana plant with staminate flowers.

Source: amsterdammarijuanaseedbank.com