It's not really true what
thecarp wrote. The main
bacteria used are
Lactobacillus bulgaricus and
Streptococcus thermophilus, but the
culture cocktail also contains more
strains of
lactobacilli and
lactococci, as well as
Bifidobacterium strains. The different types of
yoghurt are produced by different mixtures, and
ratios of the involved
bacteria, as well as the type of
milk used (the animal, low/high fat content).
During
fermentation the
lactose is fermented to
lactic acid and
polysaccharides are formed (that's the component that makes the yoghurt slimy and lumpy).
(the reasons for the high temperature during the industrial production process is, that they want to have full control over the bacteria in the milk, and has to have a longer shelf life than your home-made yoghurt.)
Making yoghurt in your kitchen can be done more easily:
just warm the milk to body temperature, add yoghurt (2-3 teaspoons per litre) and pour it into a clean
jar, close it, wrap a towel around it and leave it next to the
radiator for 3-8 hours. This even works to make
soy yoghurt.
Thing is, you can't keep on doing this. After two or three times using you home-made yoghurt as
inoculant, the
ratio of bacteria has changed, because some of them grow faster than others, hence the taste will be different.