mRNA or messenger RNA is the RNA molecule produced by the transcription of DNA. It carries the codons which are required for protein synthesis.
tRNA or transfer RNA molecule carries the specific amino acid from the amino acid pool to the ribosomes, for protein synthesis.
rRNA or ribosomal RNA are RNA molecules which are major structural constituents of ribosomes. They are involved in catalysing the formation of peptide bonds between amino acids during protein synthesis that takes place in ribosomes.