Each band represents a different population. Individual bands can be cut out and sequenced to further identify the organisms present. In the right gel you see several seawater samples. The natural bacterial community was sampled every two days for 20 days. You can see changes in the population composition over time.
Some bands disappear over time, while others get stronger.
The lower figure is a pulse field gel of a natural viral community. The different bands represent different viral genomes. The intensity of each band can be measured with a densitometer. The sample used in the figure is simply a concentrated water sample. No amplification process, such as PCR, has been used.