What is an exon? Intron?

Exon: Exon (expressed sequences) are known as coding part of the proteins present on the genes. They are found in both eukaryotes & prokaryotes. Exon is part of the nucleotide sequences present on the mRNA, that are joined together (to form a contiguous coding sequence) in the final mature RNA after RNA splicing.

Intron: Intron (intervening sequences) are known as the non-coding part of the genes. They are found in the multi-cellular eukaryotes such as humans. Intron is part of the nucleotide sequences present on the mRNA, removed by during the process of RNA splicing.