Hi Natasha,

Northern blot = RNA separation by electrophoresis
Southern blot = DNA separation by electrophoresis
Western blot = protein separation by electrophoresis

For electrophoresis, the length the band travels indicates how large the molecule is, and in some cases can also tell the nature of the molecule (ie dimerized, cyclic plasmid vs linear form etc).

The thickness of the band should be able to tell you how much protein or DNA or RNA you have, but it's not super quantitative - usually you'd compare with a different sample of the same concentration on the same gel.

I hope this helps,

Katt