From Bulbapedia, the community-driven Pokémon encyclopedia.
PokéGear is a type of gadget found in Johto. It looks like a large, advanced, wristwatch.
The main character of Pokémon Silver, Pokémon Gold, and Pokémon Crytal receives a PokéGear from their mother at the start of their journey. It has several functions:
The PokéGear displays the day of the week and the time.
The PokéGear displays a map of Johto and later Kanto.
The PokéGear will function like a cellphone, allowing the user to make calls to people that they've traded phone numbers with. This allows the player to talk to other characters and learn informaton from them, such as battling tips or when rare Pokémon appear.
The PokéGear functions as a radio. This allows the player to get tips from Professor Oak and DJ Mary on Oak's Pokémon Talk, affect how active wild Pokémon are by playing Pokémon March or Pokémon Lullaby on the Pokémon Music station, or listen to the Lucky Channel to keep track of the Radio Tower's lottery promotion.
These functions must be unlocked by acquiring special expansion cards throughout the game.
The Hoenn analog to the PokéGear appears to be the PokéNav.