Auxins are plant growth hormones. They work by making cells grow longer, and divide more.
A growth reaction to a stimulus is called a tropism, and different prefixes denote what type of stimulus is involved.
Below is a diagram showing how a phototropism causes the plant to grow in the direction of the sun.
The auxin collects on the dark side of the shoot; therefore, more growth will occur on this side of the plant. Therefore, the shoot will gradually lean towards the sun and it can now make best use of the environment.
This very affect can be seen with house plants. If you have a plant in a room with only one window, it will often develop more on the side that faces this window.
Plant hormones have many commercial uses, relating to agriculture and flower growing.
Roots: A cutting from a plant can be taken, for example a branch. If a bit of auxin is applied to the base and this is put in soil, it will grow roots and a clone is made.
Fruits. They can be used to slow down the process of ripening so they arrive fresh after transport.
Weeds. As well as encouraging life, auxin can also be used to destroy it. Applying auxins can make weeds grow too fast and therefore die.