Malt extract is sugar extracted from malted grain. It comes in two forms: liquid or dry malt extract (LME and DME, respectively). LME is a thick, sugary syrup, and DME is a dry powder.
Manufacturers use sophisticated equipment to condense malt extract by carefully evaporating much of the water. The evaporation of water from malt extract is carried out in a vacuum. The low air pressure environment created by the vacuum allows the liquid to boil at a lower temperature. This procedure is economical as well as being less pernicious to the integrity of the end product. Basically, it is sweet wort reduced to a syrup or powder. Since malt extract provides the raw materials yeast use to produce alcohol, recipes that include more DME or LME will typically have greater alcohol content and body.