The removal of introns (non-coding RNA sequences) from RNA (ribonucleic acid) is called Splicing. This process could be done by both enzymatic machinery called spliceosome and introns itself (self-splicing or Group-II). The self-splicing introns are longer than the introns spliced by spliceosome.
The man intron length of a spliceosome intron is 75-150 nucleotides and the self-splicing intron is 3413 bases (3 kb). This is because the self-splicing or Group-II has catalytic structural domains, just like protein complexes of spliceosomes.