Intrinsic semiconductors are semiconductors in their pure state and do not require doping. They have valence bands that are full and conduction bands that are emtpy at low temperatures. As shown below, the energy gap between the valence and conduction bands is very small.
The energy gap is small enough so that electrons can easily be given enough energy to jump from the valence into the conduction band, for example, by heating the material. This is why the resistance of semiconductors decreases with increasing temperaure.
For germanium and silicon the energy gaps are 0.66 and 1.11 eV respectively at 27 Degrees Celsius. When electrons jump into the conduction band they leave a deficit of negative charge or a hole, in which an electron can jump. We may think of the hole as a positive charge, and an electron jumping into the hole (leaving behind a hole at that place it came from) can be thought of as a movement of positive charge in the other direction. In this way, conduction can take place as electrons move through the conduction band or holes move through the valence band.