A Personal Digital Assistant or PDA was a device used to keep schedules, get on the internet, possibly make telephone calls, view maps or satellite images, among other tasks.
Day 1
Jamey Farrell used a PDA to communicate with Ira Gaines from inside CTU, sending and receiving messages while hidden in a bathroom stall. However, Tony Almeida caught her in the act and took her into custody as well as confiscating the device. ("7:00am-8:00am")
Jack Bauer also used a PDA with a GPS module when infiltrating the Gaines compound, and gave it to his wife Teri to help her and Kim navigate the terrain after escaping. ("11:00am-12:00pm", "12:00pm-1:00pm")
Alexis Drazen had a Sony PDA which he used to identify Kevin Carroll - he also kept detailed files of his operation on the device, which was later acquired and analysed by CTU. ("1:00pm-2:00pm", Findings at CTU)
Nina Myers also used a PDA during the sting operation with Alan Morgan. ("2:00pm-3:00pm")
Day 5
PDA destroyed by Theo Stoller after being betrayed by CTU
Jack Bauer had a PDA when observing the hostage situation at Ontario Airport, using it to receive and send messages and pictures. Chloe O'Brian was able to tap into it and send out a certain frequency, detonating the explosive vest worn by Ibrim and causing an explosion. ("9:00am-10:00am")
Theo Stoller also had a PDA, to which he uploaded the WET list before a hidden self-destruct mechanism in the data card caused it to destroy the information. ("8:00pm-9:00pm")
Day 6
During Day 6, information recorded in Lisa Miller's PDA was being downloaded by Mark Bishop and given to the Russian government. ("Day 6: 3:00am-4:00am")