This is really long, so I'll only include some of it, with a source link below:
>In Diamond and Pearl, there are 25 available apps, four of which are included with the Pokétch. The rest are not available at the start but can be received at a later time, and a few must be obtained by showing a certain species of Pokémon to another character. Listed in each section below is the in-game description of its app, followed by its in-game location and further information about it. In Platinum, the apps are ordered in the same order as they are on the Pokétch in Diamond and Pearl.
"The Digital Watch displays the current time. Don't be late for meetings with your friends!"
The Digital Watch (Japanese: デジタルどけい Digital Clock) is included in Diamond, Pearl, and Platinum with the Pokétch. As its most basic feature, the Pokétch will digitally 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. It runs off the Nintendo DS's internal clock, and through this, can keep track of the date of a Pokémon's capture as well as the time of day. There is also an analog clock app, which unlike its digital version is not available from the beginning.
When the screen is touched, it brightens slightly.
"The Calculator can handle up to ten digits. It's perfect for figuring out math questions that might pop up."
The Calculator (Japanese: けいさんき Calculator) is included in Diamond, Pearl, and Platinum with the Pokétch. It is a basic calculator, with addition, subtraction, multiplication, and division apps usable. When an answer is displayed, if the player has a Pokémon whose Sinnoh Pokédex number matches the answer rounded down to the whole number below it, the calculator will play that Pokémon's cry. Once the National Pokédex has been obtained, it will instead use those numberings.
The Hearthome Gym in Diamond and Pearl asks the player basic math questions to figure out the proper path to the Gym Leader. Getting them wrong results in Trainer battles; most of them wonder why the player didn't just use the Calculator app to get the right answer.
If the calculator attempts to display an answer that is above ten digits, or one that is invalid (like a dividing by 0 error), it will show question marks.
"The Memo Pad is useful for jotting down a note, or just doodling. There's an eraser, too, just in case you make a mistake."
The Memo Pad (Japanese: メモようし Memo Pad) is obtained in Diamond, Pearl, and Platinum from the president of the Pokétch Company after obtaining the Coal Badge. It is a notepad for Trainers to use while out in the field to draw or write (e.g., to jot down notes on the area or on which Pokémon to get to beat the next Gym), however the memo is erased if the application is changed or the game is turned off.
"The Pedometer keeps a count of the number of steps traveled. It automatically counts steps. Touch the button to reset it."
The Pedometer (Japanese: ほすうカウンター Step Counter) is included in Diamond, Pearl, and Platinum with the Pokétch. It is most useful for out-of-battle poison damage, Egg hatching, Amity Square items fetching, battling with the Vs. Seeker, and Great Marsh excursions. It is also instrumental in accessing the Surf glitch. When the counter reaches 99,999, it simply rolls back to 0.