He is capable of carrying on conversations with the crew and records everything he says on a ticker tape.
Eddie has a backup personality as well, which doesn't do much toward making the computer less irritating. This alternative personality has its own voice and concerns; a motherly personality who cajoles, berates, and acts protectively to encourage cooperation and safety among the ship's occupants. Whatever personality he takes on, however, he is thoroughly loathed by Ford, Trillian, Arthur, and Zaphod.
Eddie controls and permeates the whole of the Heart of Gold. He can manipulate navigation, defence, weapons, shielding, the Improbability Drive, escape pods, airlocks, and various other systems. He is capable of talking with other computers aboard the ship such as doors and Nutri-Matic food systems.
He has a massive computer memory but it is capable of being filled by exceptionally large requests (such as, for example, why someone would like tea). The semi-official Eoin Colfer novel And Another Thing... has Eddie replaced by Zaphod's detached second head, thereafter referred to as Left Brain. He is shown to be more competent and causes fewer accidents than Eddie (having been Zaphod's smarter head), but can still be overtaxed and frozen.