That's a hard question! Introns (intragenic regions) exist in both precursors rRNA, tRNA and mRNA. I've always heard them referred to as "spacer" sequences.

The discovery of introns lead researchers to call DNA a "mosaic" of alternating silent and expressed regions, or a "matrix" of expressed regions embedded within silent regions. This has puzzled biologists since introns are rare in bacteria, implying that they are not necessary for protein encoding and synthesis. This puzzlement has lead to a debate over whether lots of introns were present in the first cell and have been subsequently lost over time ("introns-first") or whether very few introns were present in the first cell and later invaded eukaryotes, probably through bacterial endosymbionts (mitochondria and chloroplasts) and viral infection ("introns-last").

The "introns-last" theory has gained more favor lately. If introns were introduced "accidentally" as it were, and then multiplied within the genome, they probably did not have a function in eukaryotes initially. Some people argue that they have evolved functions over time and have sort of co-evolved with the genes in which they are located. One function of introns is that by breaking up encoding regions, they allow for "alternative splicing", so that different proteins can be produced from a single gene by picking and choosing which regions to express. An analogy would be between an assembly line that, once started, could only produce one end product (a 2006 Prius hybrid, e.g.) and an assembly line that could be stopped and started to produce individual parts depending on what was needed (a new door, a new bumper, etc.). It is believed that most of the intron sequences are "junk DNA" (serving no known function). Some parts of the intron sequences are important for efficient splicing (so that the introns can be better recognized and removed).

Some introns also contain sections of genes that were once translated into protein but are no longer needed and so are cut out and discarded. Very recent work suggests that introns may even have error-checking and regulatory functions, allowing genes to communicate with each other without chemical messengers or proteins, only bits of RNA.

Intron sequences accumulate changes (mutations) much faster than exon sequences, because a mutation in the silent DNA of a gene has no effect on the gene's product or on the organism and so won't be removed by natural selection. Variations in introns are used in DNA fingerprinting and DNA profiling, in forensics, paternity and immigration cases. Introns are also used by evolutionary biologists to study the degree to which organisms are related (same mother? same population? same subspecies?).

For example, if you want to set up a forest reserve to protect a breeding population of spotted owls, you need to know how widely that population of owls roams in order to know how big to make your forest reserve. You can tag several owls and monitor them over their lifetime, which is difficult and slow, or you can locate their nests, take blood samples of the baby owls, sequence their DNA and determine how closely related they are over distances of 1, 10 and 100 miles, e.g. At some point, neighboring owls will be too far apart to interbreed and thus their intron sequences will be dramatically different. This would be the logical border of your forest preserve.

Good question! Nobody knows. This is an unanswered question in evolutionary genetics. There are (at least) two theories floating around, but we aren't sure to what extent either is correct (or if any are correct).

One of them is that these are viruses that have inserted themselves into the genome and by turning them into introns they are removed and destroyed. Another possibility is that these introns do function in someway, perhaps to regulate gene expression in some fashion, or even to be not cut out of the mRNA under some specific circumstances and thus be expressed. It is unlikely that they have no reason to exist, however; generally speaking, if life does not have a way of using something, evolution will FIND one.