Interpretation of agarose gel electrophoresis of a plasmid
The closed plasmid comes in three different forms: nicked-relaxed, supercoiled and circular single-stranded. All three forms have a different tertiary structure and thus they run differently on the gel. This should look like this (image from here, there you also find a more detailed explanation): 


When you digest the vector you linearize all the forms and they all have the same tertiary form.