A CTD is so named as it measures conductivity, temperature and depth.
Why we collect this data
These measurements provide us with basic information about the ecosystem as well as providing insight into the source of the water (freshwater from runoff or saltier water from tides). The conductivity, temperature, and pressure (depth) can also be used to determine density. The density differences between different masses of water can cause currents.
How it works
The CTD determines conductivity by measuring how well the water conducts an electric current. This information can then be used to determine salinity.
Temperature is determined using a thermistor, which measures temperature based upon changes to the electrical resistance of a metal.
Since depth and pressure are directly related, the CTD uses a pressure gauge to measure the pressure and this information can be used to calculate depth.
Image credit: http://www.sontek.com/productsdetail.php?CastAway-CTD-11