Just like any other thermometers, this is used to measure temperature, of air outside home. There are three kinds of an outdoor thermometer: digital, bulb and bimetallic strip thermometer. Each of this works differently but all gives the same result.
With regards to style, you may opt for something small and discreet or one that is huge and easily seen. There are also those that lit up at night so that the temperature can easily be read even when it is dark.
Placement though is the most important consideration because it affects the accuracy of the reading. Even if you have the best outdoor thermometer, if it is not correctly positioned, the reading that you will get is affected.