NetNavis (Network navigators, Navis, or Net Navigators) are the virtual part of the Navi-Op dynamic.
Short for 'Network NAVIgator', a Navi is a (semi-)sentient program that lives inside your PET when it is not actively trundling about on a network. Navis come in three varieties: Normal Navis, which are basic out-of-the-box navis that can be accurately described as 'semi-sentient'; Custom Navis, which are what you, the player uses, which are fully sentient; and Heelnavis, which mix the performance-tweaking of Custom Navis with the anonymity and unchangeable appearances of Normal Navis. Navis serve as a constant companion, and can do a lot of different things.
For example, they can...
-Act as a secretary, answering any calls you get.
-Tell you when you have new messages in your e-mail.
-Act as an alarm clock and/or reminder of upcoming appointments.
-Give advice on many different things.
-Act as a search engine, scowering the various networks until they find what you're looking for.
-And perhaps most importantly, Navis battle viruses.