Attenuation:  The percentage of sugars in the wort consumed by the yeast.  This varies by yeast strain and the pitching and fermentation conditions.

Conditioning:  The process of cleaning up unwanted flavors and aromas in the beer by the yeast during secondary fermentation.

Flocculation:  When the yeast begins to clump together and fall out of suspension.  This is an indicator of fermentation nearing completion.

Gravity:  The concentration of malt sugar in the wort usually measured with a hydrometer.  Gravity measured before fermentation is known as original gravity (OG) and gravity measured after fermentation is known as final gravity (FG).

Kraeusen:  The foamy head that builds up on fermenting beer.  When it begins to fall it can indicate fermentation is beginning to slow down.

Lag Time:  The period of time between when the yeast is pitched and visible fermentation begins.  The yeast are consuming the oxygen in the wort and reproducing during the lag time.

Mash:  A process of steeping malted grain in hot water that converts the starches in the grain into fermentable sugars.

Pitch:  Brewing term meaning to add yeast to the wort.

Rack:  Brewing term meaning to move wort or beer from one vessel to another.  This is usually accomplished by siphoning.

Trub:  The sediment at the bottom of the primary fermentation vessel.  It is made up of hop particulate, spent yeast and the coagulated proteins of the hot and cold break.  

Wort:  Unfermented beer made from malt extract or malted grain.  It is known as sweet wort before it is boiled with the hops.