Is there a difference in sound between these two types of strings? If so, could someone explain it?

I've got steel strings on my acoustic and when I tried to play a song similar to what I hear on the CD, it sounds totally different like my guitar has a much more metallic higher pitch sound to it.

Also, on my acoustic, what's the proper way to strum with your fingers? When I strum with my thumb, it sounds different than when I strum with my index fingernail.