A personal digital assistant (PDA) is a handheld mobile device that allows users to store, access, and organize information on the go. PDAs can be screen-based or keyboard-based, have 2MB to 64MB of memory, and prices range from $125 to $1000. They allow users to keep track of schedules, contacts, memos, budgets and more through applications like a calendar, address book, to-do list and memo pad. Additional features and memory can be added through expansion cards.