A load cell or force sensor is a device that converts force or load into an electrical signal; a set of digital bathroom scales will likely use at least one load cell. There are many different types of weighing sensors, but ours are strain gauge load cells. The load cell itself is a mechanical structure designed so that part of it deforms linearly in proportion to the force or load applied. This part of the load cell is provided with strain gauges. A strain gauge is a resistance device that changes its resistance when stretched; this converts the physical parameter (the mechanical expansion of the load cell) into an electrical signal.
Load cells are used in all applications where you need to measure a load or force. Everything from weighing a truck (on a weighbridge) to measuring the force required to insert one component into another. Load cells (or force sensors) are very often used in conjunction with LVDT transducers so that the properties of a test object can be measured (i.e. how far it extends for a given force).