PDAs

A chatter, also referred to as a COM pad is a small human computer, similar to a cellphone or Personal digital assistant, that has many different uses. They can communicate with other chatters and their users, keep calendars, and store files of different types. Novelty shops often sell disposable versions for colonial refugees, who are unable to afford a regular service.

The larger "ChatterNet", as some people call it, is maintained and policed by the Chatter Protocol Authority, and plays a vital role in everyday life. Personal finances can be tracked and managed electronically over the ChatterNet, though paper money and coins can still be used (instead of Credits). Experienced hackers regularly break into chatter lines in a process called "ghosting," permitting them to spy on other people without being noticed. Going offline is a risky venture which gives one the advantage of avoiding such surveillance, but restricts them from performing many everyday activities, similar to the Surveillance Grid, which can be very risky and inconvenient. The ChatterNet is also locked out from UNSC bases through a firewall system referred to as a "ChatterWall"; this prevented Team Durga from using their AI when they first arrived at Chawla Base.

ChatterNet is mentioned regularly throughout Hunt the Truth as an alternative to Waypoint (Communication) that isn't completely moderated by ONI and the UNSC. Benjamin Giraud uses it on occasions to contact people without the knowledge of ONI, such as Mshak Moradi and his contacts.

Trivia

 * The chatter was originally mentioned in i love bees, leaving it suspect as Halo Universe canon. However, it was legitimized with its mention in the Audio log in Halo 3: ODST.
 * The UNSC and ONI cannot directly access Chatter or ChatterNet material.

Additive
This page gives credit to the original poster on Halo.wikia.com