More than 9 million personal digital assistants have been sold.
You have been carrying around your big notebook/organizer that has your address book, daily planner, to-do lists, memo pads, calendar, project lists and expense reports -- everything that you need to keep your life organized. It's bulky, heavy and stuffed full, but if you don't have it, you're lost. But as you jot down appointments on your calendar or look up phone numbers in your address book, you see other people everywhere -- in business meetings, at the airport, around the supermarket -- scribbling things on a device that is about the size of a small calculator or maybe the size of a paperback book. They have traded their calendars and address books for a personal digital assistant, or PDA, a remarkable, tiny, fully functional computer that you can hold in one hand. And unlike that paper organizer, a PDA can hold your downloaded e-mail and play music.

PDAs are the one of the fastest selling consumer devices in history. More than 9 million hand-held computers have been sold, the vast majority of them from one company, Palm Computing. But other companies are breaking into the market. Competition means that you have more features to choose and decisions to make if you are thinking about buying a PDA. In this edition of How Stuff Works, we will examine how these devices receive information, process information, and communicate with other computers and PDAs. We will also talk about the features and trends so you can make a good buying decision.