A spliceosome is a complex of RNA and many protein subunits, that remove the non-coding introns from unprocessed mRNA. The mRNA of prokaryotes is simpler, and they do not have introns, so only eukaryotes have spliceosomes. The RNAs that spliceosomes consist of are named U1, U2, U4, U5, and U6, and participate in several RNA-RNA and RNA-protein interactions. The RNA part is rich in uridine (the U nucleotides). There are specific 5' and 3' sites on the pre-mRNA that the spliceosome recognises. The area between these locations is removed, and the two exons are spliced together.
Alternative splicing (the re-combination of different introns) is a major source of genetic diversity in eukaryotes. For example alternative splicing is what makes our immune systems so diverse (much more diverse, for example, than a simple recombination of our parents immune systems.)
Splice variants have been used to account for the relatively small number of genes in the human genome. For years the estimate was 100,000 genes, but now, thanks to the Human Genome Project we know the figure is closer to 30,000 genes. However, almost every human gene is thought to have at least two isoforms.
The term spliceome has been recently coined to describe the complete set of all possible alternative splices in an organism, in analogy to the genome or proteome. It is defined as a concept for the convenience of computer scientists working together with molecular biologists and bioinformaticians.