Intron

Introns are sections of DNA that will be spliced out after transcription, but before the RNA is used. Introns are common in eukaryotic RNAs of all types, but are found in prokaryotic tRNA and rRNA genes only. The regions of a gene that remain in spliced mRNA are called exons. The number and length of introns varies widely among species and among genes within the same species. For example, the pufferfish Takifugu rubripes has little intronic DNA. Genes in mammals and flowering plants, on the other hand, often have numerous introns, which can be much longer than the nearby exons.

Introduction
Introns sometimes allow for alternative splicing of a gene, so that several different proteins that share some sections in common can be produced from a single gene. The control of mRNA splicing, and hence of which alternative is produced, is performed by a wide variety of signal molecules. Introns also sometimes contain "old code," sections of a gene that were probably once translated into protein but which are now discarded.

While it is widely believed that most of the sequence in any given intron is junk DNA with no known function, several short sequences that are important for efficient splicing are known. The exact mechanism for these intronic splicing enhancers is not well understood, but it is thought that they serve as binding sites on the transcript for proteins that stabilize the spliceosome. It is also possible that RNA secondary structure formed by intronic sequences may have an effect on splicing.

The discovery of introns led to the Nobel Prize in Physiology or Medicine in 1993 for Phillip Allen Sharp and Richard J. Roberts.

Some introns such as Group I and Group II introns are actually ribozymes that are capable of catalyzing their own splicing out of the primary RNA transcript. This self splicing was discovered by Thomas Cech who shared the 1989 Nobel Prize in Chemistry with Sidney Altman for the discovery of the catalytic properties of RNA.

Intron evolution
There are two competing theories as to the evolutionary origin of introns, which is usually studied in a highly conserved family of genes such as the actins. In the introns-early model ancestral genes are believed to have included a large number of introns, some of which have been lost over evolutionary time, leading to the different but similar intron patterns in related genes of different species. The introns-late model suggests instead that introns occur in the same location in variants of a given gene because the location is in some way predisposed to the introduction of an intron, and therefore that a similar intron pattern may arise in two different species by a form of convergent evolution.

Evolutionary "change in intron–exon structure is gradual, clock-like, and largely independent of coding-sequence evolution." 

Identification
Nearly all eukaryotic nuclear introns begin with GT and end with AG (the GT-AG rule).

Reference

 * 1) Walter Gilbert (1978 Feb 9) "Why Genes In Pieces?" Nature 271 (5645):501.