Gel electrophoresis analysis
After electrophoresis, the different fragments are visualized as bands at specific distances from the top of the gel (the negative electrode end) on the basis of their size. The sizes of the nucleic acid samples can be estimated by comparing the distance with the molecular weight standard samples (also called DNA ladder).
Each fragment will be a band in the gel. A mixture of fragments of varying sizes appears as a long smear. If the fragments are too large, e.g. uncut genomic DNA, they will form a single large band at the top of the gel.
Figure 1: A completed gel electrophoresis experiment with 5 samples and the DNA ladder on the right. The DNA ladder can be used to estimate the sizes of the bands in the samples.
Circular plasmids move at different speeds depending on their conformation (nicked, linear, covalently bound, supercoiled, and circular single stranded). A supercoiled plasmid travels faster because it has less friction against agarose matrix than nicked or linear plasmid.
If you want to isolate a specific fragment you can cut the gel and extract the DNA.