how do I calculate concentration of plasmid DNA based on gel electrophoresis?

Take 1 microliter of your plasmid DNA, digest it, and run it in lane 1 of the gel. This 7 kb plasmid is digested into 3 fragments: 4 kb, 2 kb, and 1 kb. Run the DNA markers of known concentrations in lane 2. You find that the intensity of the 2 kb band of your plasmid is similar to the 20 ng marker. Calculate the concentration of your plasmid DNA.

What does splitting dna tell a researcher, why is it important? What is the purpose of isolating a sequence?

I just wanted to know, because i have done gel electrophoresis, but i don't understand what the results are telling me, why do i need to separate dna?

How is a DNA microarray different that gel electrophoresis?

I know that they are both used to kind of sort DNA and whatnot, but what's different?