Diamond and Pearl are the fourth addition to the Pokemon games. They came out for the DS, and made a splash in the world of Pokemon. The new games include the new features of GTS or the Global Trade Station, and being able to go into a trade room to trade, battle, draw and more. Storyline
Your goal is to see all 150 Pokemon in the Sinnoh region. Once you have seen them and all have defeated the Elite Four,

you are given the National Pokedex, which is a device that you need to complete. To complete the National Pokedex, you must have information on all the Pokemon in the Pokemon games. Once you have the National Pokedex, the Pal Park will open; this is a place where once you have 6 Pokemon in another game such as Firered, Emerald, or Sapphire you can transfer it to your Diamond/Pearl game. Doing this will allow you to go into the Pal Park to catch the Pokemon, in which the 6 Pokemon that you have came from.
Your other goal is to defeat the evil Team Galactic. They are out to make a new world, and to steal the Pokemon's power. You need to defeat them to help finish the game. After meeting them at Mt. Coronet, they will fortunately leave your life.
Pokemon Diamond

Unlockable: Elekid
Insert Fire Red in the GBA slot.
GBA Cartridge Extras
Unverified. Insert the GBA cartridges to find the following Pokemon (you still need to find and capture them yourself).
Ruby
Seedot -- Hakutai Forest, Route 203, Route 204
Nuzleaf -- Route 229
Mawile -- Iron Island
Zangoose -- Route 208
Solrock -- Thrust Lake, Thrust Lake Surroundings, Wisdom Lake, Aspiration Lake, Tengan Mountain, Departure Springs, Return Cave
Sapphire
Lotad -- Route 203, 204, 205 North, 212 South
Lombre -- Route 212 South, 229
Sableye -- Iron Island
Seviper -- Route 208
Lunatone -- Thrust Lake, Thrust Lake Surroundings, Wisdom Lake, Aspiration Lake, Tengan Mountain, Departure Springs, Return Cave
Emerald
Pineco -- Hakutai Forest, Route 203, 204, 229
Gligar -- Hard Mountain, Route 206, 207, 214, 215, 227
Shuckle -- Route 224
Teddiursa -- Route 211, Wisdom Lake
Ursaring -- Wisdom Lake, Route 216, 217
FireRed
Caterpie -- Route 204 South
Metapod -- Hakutai Forest
Ekans -- Route 212 South
Arbok -- Safari Zone
Growlithe -- Route 201, 202
Elekid -- Route 204 South, Powerplant
LeafGreen
Weedle -- Route 204 South
Kakuna -- Hakutai Forest
Sandshrew -- Lost Cave
Sandslash -- Route 228
Vulpix -- Route 209
Magby -- Hard Mountain, Route 227
Any Pokemon GBA Cartridge
Haunter -- Forest Mansion
Gengar -- Forest Mansion Room with eyes
Unlockable: National Dex
Finish the Shinou Dex.
Unlockable: Pal Park
Acquire the national dex.
Unlockable Legendary Pokemon Heatran
This legendary can be male or female but cannot breed. Get your national dex and go to the island in the north and east. When you get to Stark Mountain, you must team up with someone to go through the place. The trainer will take the item and depart. Head back to the Survival Area, and your "partner" will be inside a house. Talk to her and she'll leave.
You have only one chance to capture Heatran, so save the game first. Return to the mountain, and Heatran will be waiting. The pokemon is at level 70. Heatran can learn the following moves:
Level 1 - Ancientpower Level 9 - Leer Level 17 - Fire Fang Level 25 - Metal Sound Level 33 - Crunch Level 41 - Scary Face Level 49 - Lava Plume Level 57 - Fire Spin Level 65 - Iron Head Level 73 - Earth Power Level 81 - Heat Wave Level 88 - Stone Edge Level 96 - Magma Storm
Unlockable Eevee
Get an Eevee through one of two ways:
#1 Receive an Eevee from someone named Mizuki in Hearthome City
#2 Catch yourself a wild Eevee in the back of the Pokemon Mansion
Had info help by: http://en.wikipedia.org/wiki/Diamond_and_Pearl