Strain guages  as the name suggests helps you measure the strain on a surface. There are various types of strain guages based on the technology of measuring the strain. They are

a) Foil strain guages

b) Semiconduuctor strain guages

c) Optical strain gauges

  Foil strain guages are the most popular and  primarily consists of a very thin metallic foil etched out to have a finite resistance. This foil is encapsulated in an insulator such as an epoxy and the whole device is bonded on to the surface on which the strain is to be measured. The change is strain causes the foil to elongate or contract by which the resistance of the guage changes. The change is resistance is proportional to the strain. The foil of strain guages are made of few alloys such as constantan and Karma. The thickness of these guages are in a few microns and dimensions as low as 1mm x 1mm.

Semiconductor guages  use a similar philosophy, however, the change is  in conductivity of the semiconductor due to change in  strain. These are not very popular due to the high cost and also due to the thermal drift of the semiconductor material.

Optical strain guages is an emerging technology which uses the change in the direction/ quantum of light due to the change in refractive grating due to strain.

 Foil strain guages being most popular is used primarily for force measurements. They are used in either quarter bridge, half bridge or full bridge configurations. Being used as the arms of a Wheatstones bridge, there are multiple advantageous like temperature compensation, zero balancing etc.