The PokéGear was introduced in Gold, Crystal, and Silver. It hasn't changed too much in terms of features. It has a Phone, a Map, and a Radio.
After some battles trainers will give you his or her phone number. These trainers may find items, may want a rematch, or may just give you some helpful tips on a sale.
The Pokegear also shows a map. This map is a bit more detailed than in previous games, it even shows town layouts on the places you hover over. Just like the DP marking map, it also shows the locations of roaming Pokemon, like Entei and Raikou. You can add notes some way or another now as well.
The PokeGear has a Radio, just as in Gold, Silver, and Crystal. This radio can be played while walking.