Yes, looks like aphids. There are lots of different kinds. I had the same as you on my turnip leaves. Hard to remove after they get that populated!

It is shed aphid skins, the white ones.

How can I get rid of them?

Probably too late now as the infestation has gone on far too long and too much damage has been done. But there are literally 500 discussions here about "how to control aphids" the search will pull up for you.

In the early stages you can just hose them off with the water hose, strip them off with your fingers even. At this point one of the specific recommended pesticides for aphids is likely the only option. But the plants may not recover from the damage in the photo.

http://www.gardenweb.com/gardenweb/query/aphid-control/topic=cornucop

Dave