In weaving, weft and warp refer to the two sets of thread that are woven together to produce fabric. The weft refers to the threads that run horizontally on the loom and get woven in front of and behind the warp. The warp refers to the threads that are strung vertically on the loom.
Because the warp is stretched taut on the loom, it typically needs to be made from a stronger, courser material than the weft. Wool, linen, alpaca and silk are all examples of strong fibers that make good warp threads. The warp provides the structure for the fabric and is what gives fabric its strength and form. Because the warp is tightly stretched during the weaving process, fabric does not stretch any further along the warp in its finished form. Fabric may stretch along the weft.
Weft threads are subjected to less strain in the weaving process and are, therefore, generally chosen for their appearance rather than their strength. The way in which the weft is woven in and around the warp determines the look of the finished fabric. Variations in pattern, texture and color in traditionally woven fabric are all created by the weft. Weft threads are sometimes referred to as "filling" thread because they make up the body of the fabric.