While reading up on souring bacteria I ran across an article from 2005, written by Tomme Arthur of Lost Abbey Brewing. You can find the article here, it is well worth your time to read it. The title of the article is "A Perspective on Bacteria and Wild Yeast."
He mentions the different wild yeast and bacterias used for creating sours (though focuses mostly on the different versions of Brettanomyces in the article as well). While in a spontaneous fermentation there are quite a few different strains of wild "critters" involved in the process, cultured strains tend to fall on these different microorganisms.
Brettanomyces: is a power house that tends to be used more than others since it was originally cultured. Some flavors associated with this one are smokey, spicy, horsey, or even barnyard. An interesting tidbit: according to the wiki this was also the souring agent used in Guinness before they changed their formula in the 80s. This was done by adding a portion of old stout in with new stout before bottling.
Lactobacillus: This bacteria is best known for naturally fermented foods such as sauerkraut or yogurt. It can live saltier environments than many harmful bacterias, which is why it has been a strong force in food preservation, many bacterias die in the acidic environment it creates. This bacteria is part of the natural flora in the human digestive system. When you have run a course of antibiotics it is a good idea to eat foods like yogurt that still have life cultures. These live cultures will replenish the good bacterias in your system that the antibiotics have killed off.
Pediococcus: This is a good bacteria that works with Lactobacillus. A flavor that it brings to beers and even wines is a buttery or butterscotch note. In a non-sour beer this can be a sign of an infection (an infection is when you don't want these flavors present) but it is something that can add to the complexity of a sour beer.
Acetobacter: This bacteria turns ethanol (the alcohol in wine and beer) into acetic acid. The main purpose of this bacteria is the creation of vinegar. All vinegar comes first from a form of alcohol. When this bacteria infects a wine it can make it too acidic for drinking. When it infects a sour beer it can add to the complexity of the beer overall.
Time for a pint...