Muscle wire, also known as smart wire, more commonly known as nitinol wire, which is the name for very high performance, shape memory alloy, actuator wires. It is made of nickel-titanium.
Smart wire can be used in electronics, cars, medical and in smart materials. It can expand or contract, ideal for smart textiles and model-making projects. On passing a current through the composition of the wire changes which makes it contract. This movement can be increased using levers to produce moving parts within models.
Light Stitches muscle wire has fitted crimped copper ring terminals to make model making simple. The wires can easily be soldered on to the copper ring terminals. It can be repeatedly used.
Movement can also be controlled using a 555 timer circuit with a supply voltage of 4.5V, this allows the correct 3V to operate the smart wire. Movement can also be controlled using PIC chips to activate and de-activate the muscle wire using the correct 3V supply.
Further information on muscle wire can be found here. Source Wikipedia
Need further explanation of what smart wire is and what it can do please watch the video from Cool Components/Spark Fun.