Exon

When mRNA is produced from eukaryotic DNA by RNA polymerase, primarily the DNA transcribed creates 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 - the introns are removed, the exons are spliced together and a poly-A tail is added.
The mature mRNA is then suitable for translation.

Prokaryotic DNA only have exons. Eukaryotic DNA include introns and exons.