Splicing occurs in the nucleus of a cell. DNA contains regions called introns and exon.
The Introns have to be removed because if they weren't they would be translated into non-functioning proteins. A process called RNA splicing removes these introns and splices exons together.
snRNAs form an RNA protein complex called a spliceosome that cuts the pre-mrna and releases the introns and joins the ends of the exons together to produce mature mRNA.
Alternative splicing is a process for generating different mature mRNA from a single gene by splicing together different sets of exons during RNA processing