The warp is the lengthwise core support of a fabric. They tend to be stronger and more coarse to hold up to tight stretching. They form the base upon which the pattern of the fabric is woven and give it body and form.
The weft (sometimes called the woof, although this is a somewhat archaic term) is woven in between the warp threads to create various patterns and textures. The weft is sometimes considered "filler" thread.
The weft is woven through the warp by means of a shuttle. A shed, a vertical space between the raised and unraised warp threads, is formed to allow the shuttle to carry weft yarn through. A single crossing of the weft from one side to another is called a pick.
The word weft is derived from the Old English word wefan, meaning "to weave," while the word warp come from an Old Norse word varp, meaning "the cast of a net." In other words, the warp acts as a net to hold the weft so they will not escape and unravel.
|White warp with blue weft|
All designs are created by weaving the weft yarn over and under the warp threads in a repeated pattern. Pretty amazing when you stop to think about it...