Serebii.net has a quick explanation of how you’ll be transferring Pokémon over from previous games to Pokémon Black & White. This is done through the use of an ingame feature called the “Pokéshifter.”
Located somewhere within the Isshu region is a building — similar to the Pal Park from Hg/Ss and Di/Pe/Pl — that will allow you to connect with previous games, provided you have two DSes. Once you connect, an option will appear in the Gen IV games to allow you to complete the transfer. You’ll be able to move six Pokémon at a time, although, there’s no mention of whether there are any limits per day (or any other timeframe).
Capturing the monsters on the Black & White end involves a minigame where they’ll pop out of a set of bushes on the DS’s top screen. Using the touchscreen, you’ll need to aim a reticule to shoot a Pokéball with a bow (a…Bowkéball?) to capture them.
The minigame in Black & White will require you to adhere to its time limit.
Pokéshifter minigame image sourced from Serebii.net.