Encryption (along with the reverse process, decryption) was a means of transforming information so it would be readable only to someone possessing the key, which was then used to transform the information back to the original text.
When Nina Myers was trying to trace Phillipe Darcet's finances, she found that his account number was encrypted. Jack Bauer told her to put Tony Almeida on it, who managed to break the encryption and trace the money. ("12:00am-1:00am")
The key card that Richard Walsh gave to Jack was full of encrypted information. Jamey Farrell was given the task of decrypting it, and she found the address 18166 San Fernando Road before a lockdown rendered her unable to access CTU's decryption software. Later Milo Pressman was called in to take over from Jamey, and he started using an S-box inversion on the card. He discovered a list of plastic surgeons that he believed were used to cover someone's identity. However Jack, under orders from Ira Gaines, switched the cards and Milo's decryption no longer worked. Later the card was sent to the archives. ("3:00am-4:00am", "5:00am-6:00am", "6:00am-7:00am", "8:00am-9:00am")
While going through Jamey's computer, Tony discovered an encrypted e-mail with the subject Ted Cofell still visible. He put Milo onto decrypting the main text, and at 9:45am Milo cracked it and discovered that a wire transfer was made to Ira Gaines at 2:10 that morning. ("9:00am-10:00am")
The Blowfish algorithm used to encrypt files on the computer of terrorist Sabir Ardakani was discovered by CTU Los Angeles intelligence agent Chloe O'Brian. She claimed that CTU could crack this encryption method.