The process of gene expression always starts with the transcription, the production of an RNA copy from the DNA. During this process the RNA polymerase adds nucleotife after nucleotide to the growing RNA strand according to the DNA template. The only difference is the exchange of thymines (T) by uracils (U) within the RNA sequence. After the production of the primary RNA transcript (pre-RNA) in eukaryotic cells, the RNA has to undergo several modifications. Two major modifications are the 5´capping and the 3´cleavage and polyadenylation which is crucial for mRNA export into the cytosol and translation initiation. Another very important modification is the RNA splicing in which introns get removed from the RNA sequence and necessary exons get spliced together. Alternative splicing is a variation of this process where some introns or exons can be retained and create an alternativ RNA sequence from the same gene. This allows the production of different proteins from the same genomic sequence and increases the size of the proteome.
In some cases, the mature RNA also represents the final product. However, in most cases the final product is a protein which needs to be synthesized from the mature mRNA during translation.