A bacterial culture which produces lactic acid.
A small volume of wort mixed with yeast. Used to increase the yeast population before introduction into the main wort. Liquid yeast cultures should always be used with a starter, as the population is not nearly sufficient to achieve ideal fermentation. Use of 2 packages of dry yeast gives sufficient population such that a starter is not required.
A little bit of beer fed to the yeast a few days before brewing. The idea is to get lots of yeast, so you feed it some beer and then step up the starter every day or two by feeding it more beer. By brew day, you've got a big starter (lots of yeast) that you pitch into the cooled wort. Big starters ensure that the yeast takes over before any air borne bacteria or wild yeast can get a foothold.