In prokaryotes, mRNA is produced by splicing a large primary transcript from a DNA sequence. The mRNAs of prokaryotes are usually very short lived (from seconds to more than an hour) and protein synthesis starts even while the mRNA is still being synthesized. The resulting mRNA is essentially mature upon transcription and requires no extensive processing.
In eukaryotes, the mRNA is produced in the nucleus (during transcription). This mRNA needs to be processed extensively to become mature. This extensive processing includes the addition of a 5' cap at the 5' end and a sequence of adenylate groups at the 3' end, the poly a tail, as well as the removal of any introns and the splicing together of exons. When the pre-mRNA has been completely processed, it is now called a mature mRNA, which will then be transported for translation into the cytoplasm through the nuclear pore. The eukaryotic mRNA has relatively longer lifespan, e.g. mammalian mRNA can live from several minutes to days.