A culture can be done on ANYTHING... a plate, a toilet seat, a phone, urine, spit, blood etc. A swab of the surface or a sample of the liquid is swabbed onto growth mediums in petrie dishes and left to grow in perfect growing conditions for 24-48 hours (or longer).
Then as others have mentioned the dishes are then examined for visible growth, and microscope slides are made of the various growths and examined to determine what the growth is. They might even possibly do an antibiotic culture to determine the best antibiotic for the bacteria present.
For more information and pictures:
http://www.mclno.org/labpartners/mic...ial/sld001.htm