Surfactants are most essentially molecules with both polar and non-polar parts. This enables the different ends to interface between differenc substances, for example detergent molecules interfacing between oil and water.
As usual, organic versions tend to be far more complex and have more interesting behaviour. An example is the lining of the alveoli in the lung, which enable the surface tension in the sections to change according to the stage of inflation. This ability to vary surface tension is vital to prevent the alveoli from rupturing or collapsing.