The messenger RNA (mRNA) is single stranded, linear basically. It only constitutes 2-5% of the total RNA.
The function of mRNA is to take the information from the DNA, which is in the nucleus, and take it to the cytoplasm, where the ribosomes are where the proteins will be synthesized with the amino acids provided by the tRNAs.
mRNA is formed from a strand of DNA in a process called transcription. With the complementary nitrogenous bases, a template is created with the genetic information necessary for protein synthesis. The size of the mRNA depends on the size of the protein for which it carries information. After performing their function, protein synthesis, ribonuclease enzymes destroy it to prevent unnecessary protein production. When the synthesis of a particular protein is needed again, new mRNA will be created.