Pokémon Gold and Silver are continuations of Green, Yellow, Red, Blue. In these versions you will have a new places to explore, and there will be about 250 pokémon (100 new ones). (Rumors have it you don't play as Ash this time) The new Pokémon will have some new moves/abilities (so it won't be moves from the red/blue versions recycled) You can still swap the gold and silver Pokémon with red/blue cartridges, however, the new pokémon won't work in the red/blue cartridge. For example, there is an evolved form of Slowbro called a Slowking or something (it is pronounced Yadokingu in japanese) in the gold game. If you trade it to a blue/red cartridge, it would be a Slowbro instead of the evolved form. It gets "downgraded."
There will also be different sexes for each pokemon, like the nidoran from red/blue. This means breeding! You can also cross breed and produce variations. Another new feature is the concept of time. The game will have a timer so there will be nocturnal Pokémon who are only active during the night and you probably have to wait for some pokémon to wake up... now that's cool! There will also be a skateboard now as a mode of transportation. Nintendo is also taking the time to make this game in of color to take advantage of the Gameboy Color. These games are scheduled for released here in North America in maybe the first half of '00

New Pokemon

The regular screen
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Battles
Buru VS. Mariru
Butterfree VS. Venusaur
You VS. Schoolgirl
Charizard VS. Donfan

Pokedex
Clefairy
Farfetch'd
Slowking

Most of the info and pix from the Psychic Pokemon Network.