How are people getting Pokemon from past generations onto new generations?

I wasn't sure where to put this, hopefully I'm putting it in the right place. I tried looking for other threads with this question, I didn't find any. I apologize if I made a mistake.

BUT ANYWHO...
I was curious how people are getting Pokemon from past generations onto new gens. In the few days I've been here, I've had a couple very nice people trade me a Torchic, Wartortle, & a Charmander. I was curious how its being done, if there was a way I could do it myself so I don't have to keep asking; as I don't have much to offer yet in my Black 2 since I've only had it a week. I wasn't sure how this is being done. And even if I can't do it myself, I'm just kinda curious. Does it have to be traded to my Black 2?
Thank you.

Use poke transfer or pal park. If u want to make a hack use pokegen

This would actually belong in this thread, but I can answer the question for you. With an original DS or DS Lite, you can put a third generation game in at the same time as a fourth generation game and transfer the Pokemon over. From fourth to fifth generation, you need two DSs but you can still transfer them in between. You can't transfer any Pokemon from the first two generations.