RNA (ribonucleic acid) is a form of genetic material similar to DNA (deoxyribo nucleic acid) but differing in that RNA contains the sugar ribose not deoxyribose. RNA also differs in that it contains the base uracil in place of the base thymidine found in DNA. Due to these differences it invariably is unable to form a double helix as DNA is, but is able to pair with itself, forming complex secondary structures which aid it's function.
mRNA: RNA that is translated from DNA and forms the template by which protein is created by the ribosomes in a cell.
tRNA: Couples the code of the mRNA strand to the concurrent amino acid sequence being created on the ribosome.
rRNA: together with protein, rRNA forms the constituent parts of the ribosome.