Force Sensitive Resistors (FSRs) are sensors that allow you to detect the pressure exerted on them. They're similar to a potentiometer (like in CIRC08), except instead of varying resistance by twisting, the FSR's resistance varies with pressure. 

The FSR is made of 2 layer separated by a spacer. The more you press, the more dots on the active element touch the semiconductor, and that makes the resistance go down. They're not good for detecting exact weight, but they're great for detecting squeezing and pushing and poking. If you'd like to dive a bit deeper into how FSRs exactly work, ladyada has a great learn guide which goes over more technical details.