A mobile device is simply a portable computing device such as a tablet, PDA, or smartphone.
Mobile devices come in a number of formats, but their most important characteristics are portability, computing power, and network connectivity. Network connectivity can be through cellular networks or wireless connections.
Mobile devices can have touch screens, qwerty keyboards, phones, etc. built in to them.
Smart phones and tablet computers are the most commonly used mobile devices and the usage of these mobile devices is growing rapidly. In fact, according to the Pew Internet report, Mobile Health 2012 as of November 8, 2012, "Fully 85% of U.S. adults own a cell phone. Of those, 53% own smartphones."