Originally from http://hdtvantennabest.com/best-hdtv-antenna-indoor-outdoor/
If you are staying miles from the broadcasting tower and you are having a lot of obstructions, then you can go for outdoor antenna.
Outdoor antennas have larger reception and amplification capability but at the same time they are larger in size and takes larger space to install. Outdoor antennas are made for best performance.
Indoor antennas are smaller and easier one to install and are available in number of models. In most of the cases you will be having only one option, indoor antenna.
More at Indoor and outdoor antennas