Carapils adds body, mouthfeel and improves head retention by adding dextrines, proteins, non-starch polysaccharides, and other body-building compounds into your wort. Carapils is usually used at a level of 5% or less of your total all-grain malt bill. However it can be used at a higher rate in special cases, say if you are making a Session IPA, and are trying to simulate a higher gravity beer. Because of its extremely light color and neutral flavor it won't have much affect on the final color or flavor of your beer. Carapils belongs to a category of malts labeled as Dextrine, sometimes Dextrin, malts. We mention this because you might see either of these names in your recipe.
You can steep Carapils in an extract batch but will only get about 20% utilization, or 20% of the goodies of this malt dissolved into your beer. While not as complex as Carapils malt, for extract brewing some brewers prefer to use dextrin powder.