Image source: http://www.crescent.se - Crescent Logic LTC800
Image source: http://www.giant-bicycle.com
Image source: http://www.nishiki.se - Model 352
Hybrids have made their apparition in the 90's. When they first were put on market, they rapidly gained interest among buyers looking for a polyvalent and comfortable bicycle. The hybrid bicycles filled a gap that was present prior to the 90's, where only conventional road bicycles were available for casual biking.
The main drawback with conventional road bicycles was the same drawback that race bicycles have, the too arched sitting position. Casual riders needed a bicycle that would allow a more comfortable sitting posture. The all terrain mountain bicycles, that already existed at that time, had that characteristic. So, one day, someone came with the idea to merge the best of both types of bicycles and came up with the hybrid concept. Indeed, if you take a quick look at an hybrid bicycle, you notice it looks like a lightened MTB. Since then a lot of variations were made from the original hybrid concept. As you can see from the pictures above, there now exist performance hybrids (first picture of this section), casual (middle picture) and comfort-minded hybrids (last image).
Depending on the specific needs of a cyclist, one of these hybrid sub-categories will be best.