Mobile computing is the culmination of a number of technological advances that have occurred in parallel. Key aspects that have led to the current explosion of mobile computing include general computing, user interface, display, communications, storage, battery, and culture. I've collected some of the key milestones into the above timeline for consideration. (You can click the image to open a larger version). Please let me know of any items worthy of inclusion or alteration.
Mobile computing is used when there is a need for computing power away from a desk or office environment. Initially this became evident in military and advanced transportation scenarios. For example when calculating ballistics in ships and submarines or to help carry out checklists on spacecraft. These days with mobile computing being ubiquitous everyone can use the power of mobile computers to assist with business and personal tasks.
Between the abacus and the pocket calculator there weren't a lot of leaps directly in mobile computing. However the complimentary technologies where falling into place to support the current mobile computing that we enjoy today. Also I find interesting that throughout the years inspiration in the form of science fiction predicted or prompted advances in technology. For example Star Trek is credited as inspiration for the development of the mobile phone. Famously the film a space odyssey 2001 was referenced in law suits in relation to the development of the tablet computer.
Now with growing power and sophistication mobile computing is often discussed in terms of being the new desktop or the new internet. For more thoughts on this and other topics please refer to my blog on the future of mobile computing.