Colorwork is any technique that uses two or more yarn colors in a single piece to create patterns or motifs. In knitting, the main styles are stranded (fair isle) colorwork and intarsia; crochet has tapestry crochet and the mosaic technique.
Stranded colorwork carries the unused color along the back of the work, while intarsia uses separate bobbins for blocks of color with no floats. Colorwork is where a lot of makers' favorite sweaters, mittens, and yokes get their striking graphic look.