Exon

When the transcription of eukaryotic DNA by RNA polymerase occurs, the DNA transcribed produces pre-mRNA which includes both introns and exons.

Introns are non-coding elements of DNA (however, often have a regulatory function). Exons are the protein coding regions of DNA. 

Pre-mRNA is processed into mature mRNA by the removal and splicing of introns followed by the addition of a poly-A tail.  The mature mRNA is then suitable for translation into the coded proteins. 

Prokaryotic DNA only contain exons, whereas Eukaryotic DNA contain introns and exons.