Genus of bacteria that eat sugar and produce lactic acid. Extremely common and often symbiotic with humans.
Widely used for fermentation ("lactofermentation"). Part of the team for e.g. kombucha, sauerkraut, kimchi, sour beer.
Also important in pickling: lactobacilli are salt-tolerant, so they will live happily in brine and produce lactic acid as usual. The salt/acid combination is deadly to most other microbes, and very tasty to humans.
Interaction with yeast is often of interest. Lactobacillus tends to dominate yeast in warm, acidic environments. In the human body they can suppress yeast infections while living on absorbed nutrients. They are frequent spoilers of beer, if acidity/temp get out of control.