Pokemon Diamond and Jade are bootlegs of the game Keitai Denjuu Telefang 1 Power and Speed versions (Diamond being Power and Jade being Speed).
The game plays like a typical Pokemon game with a few differences. Instead of having a party of six creatures (which are called Denjuu in the original game), the player will have one Denjuu with them and in battle they can use a mobile phone like device (called a D-Shot in the original) to call in up to two more Denjuu depending on the D-Shot's coverage. Some Denjuu will be farther away than others meaning it may take multiple turns for far away Denjuu to arrive.
The bootleg version plays the same but it's riddled with bugs. Bugs include saves not loading on certain cartridges, the color pallet in the intro being glitched, and dialing "secret" Denjuu causes the game to crash. The game also has a notably bad translation containing many bits of "engrish" such as "Some Points of 4 lost!" and Denjuu users, called T-Fangers in the original, are called things like T-Mildew, Milde-T, and Mildew-T. The translation also has an abundant amount of swearing.
The "Pokemon" in this game are creatures called Denjuu which a T-Mildew can call by befriending them and getting their D-shot number. Each Denjuu has a habitat type, Forest, Aquatic, Desert, Grassland, Mountain,and Sky and learn moves of that type. Depending on the other Denjuu's type they can do more or less damage. Denjuu also have three distinct ways of evolving being natural evolution (basic level up), fusion evolution (Fuses Denjuu with man-made object, the quickest way the evolve but typically gives worst stats though some Denjuu can only evolve this way), and lab evolution (Evolves Denjuu from specific DNA samples, tends to be better stat wise).