"Junk DNA" is a term applied to DNA that does not serve a structural purpose and does not belong to a gene (including introns) and accordingly is not expressed as proteins in the day-to-day operation of the cell.
The precise percentage of DNA in a given genome acting as "junk" is up for debate, but junk DNA does appear to be relevant; for example, certain organisms have much larger genomes than humans, yet they also have a higher percentage of junk DNA.
