Most are approximately 3 inches by 4 inches by about three quarters inch thick and operate on batteries for various lengths of time. Most are like the Palm Pilot and use a stylus to make the unit function. Some are like very small laptops, complete with keyboards and function like laptops. Many PDAs have slots in which PC Cards or other modules can be inserted to do special functions or make calls. Nearly all of them have some sort of PC serial, USB, or other connection.
Data storage is minimal with some providing extra modules for storage.
External keyboards are available for some units. Mobile phone connections are available for some units. The add-ons can add up in price. PDAs are sold in computer stores, business office supply stores, computer catalogs, wireless phone catalogs, the WWW, and just any other place you go into. Look for price, connectivity, and do some research on the third party vendors who make things for the basic device.