PDAs are handheld devices that link to a personal computer to download and upload (synchronize) data, typically a list of contacts, schedules and notes. They come in many forms: there are palm pilots, treos, smart phones, blackberries, windows mobile devices. They all have the capability of accepting input, storing, retrieving and uploading data. Some have a wireless communications via cell phone service or WI-FI built right into them. Others, designed for inventory purposes, have barcode scanners in them. See the video in this section for an example of that feature in use in a door knocking volunteer management application.
Using a PDA in a voter project entails modifying it with some custom software and pairing it with a computer that has access to the canvass database. The typical system downloads contact information for a district to be walked. The canvasser takes it along and uses it to record responses received at the doorstep. The software on the PDA has the capability of accepting voter canvass information such as issue preference or voter registration status and then uploading it to a central database. The upload is usually done via a direct wire connection, but it can also be done via a wireless connection.
As Anthony comments, it's an expensive proposition to outfit a large number of walkers with PDA's. Costing more than hundred dollars each, there is also a challenge maintaining the inventory, repairing them and keeping them charged. For these reasons, they are out of reach financially for most organizing groups.
If you decide you can afford PDA's for you campaign, you'll need to do a search for software or software developers, who may not be easy to find. To get you started thinking about what the software might do, here's a link to a description of an application that Voter Activation Network built in 2004.