From Bulbapedia, the community-driven Pokémon encyclopedia.
The Pokétch, sometimes identified as the Pokétchi, and short for Pokémon Watch, is the navigational item that will be present in Pokémon Diamond and Pearl, much like the PokéGear and PokéNav before it.
It is rumored that the Pokétch will have up to twenty functions, most likely including a map of Shin'ō like its counterparts did for Kanto, Johto, and Hoenn, however, only fifteen of its functions are known at this time. With rumors that there is yet another Radio Tower in Shin'ō, it may also have a radio function.
As its most basic feature, the Pokétch will display the time of day, much like the PokéGear of Johto had, only this time running in 24-hour format, instead of 12-hour AM/PM format. Assumptively, it will run off of the Nintendo DS's internal clock, and through this, may even keep track of the date of a Pokémon capture as well as the time of day. There are two different watch functions, an analog and a digital.
Type matchup chart
This function will allow a trainer to choose the type of attack and the type(s) of the opponent Pokémon to determine whether a move is super effective, normal, not very effective, or ineffective against an opponent.
Breeding center monitor
Unlike previous generations, where trainers would need to stick around the breeding centers to make sure that they got their Pokémon egg as soon as it was ready, the Pokétch allows trainers to see the levels, genders, and species of the Pokémon located in the center, and will show whether or not an egg has yet been produced.
The Pokétch also has a basic calculator, with addition, subtraction, multiplication, and division functions usable. It is able to display up to ten digits.
This device allows trainers to count the amount of steps taken, a value that often determines things, such as out-of-battle poison damage, egg hatching, and Safari Zone excursions.
A notepad for trainers to use as they walk the field, for writing notes on the area or notes on which Pokémon to get to beat the next gym.
This shows the most basic stats of the Pokémon that the trainer has with them: HP level, species, and whether or not they are holding an item.
Shows the tameness of the various Pokémon in the party.
Finds items located on the top screen of the DS when touched.
A map of Shin'ō showing where mature berry trees are located in the region.
Shows the twelve most recent Pokémon captured, evolved, hatched, or traded into the game.
Counts anything players choose to; upon press of the bottom screen, the number goes up by one.
The version of the Template:I for the Shin'ō region. Allows players to place different markings on locations to show them as destinations, as well as the locations of the two Pokémon that travel around Shin'ō, Crescelia and Emurit, much like Entei, Raikou, and Suicune travelled Johto and Kanto and Latios and Latias travelled Hoenn.
Allows interaction with other players using the functions nearby.
Players can flip a coin.
Shows the three most recent catches in Pal Park.
Serebii.net's section on the Pokétch.