The Central Dogma of molecular biology is: DNA-->RNA-->PROTEINS. This means that all an organisms genetic code is coded in the language of DNA, as nucleotides. Then, when a gene is warranted, the DNA is "transcribed" into the same language of nucleic acids, but a slightly different molecule, RNA. DNA is composed of nucleotides with the sugar deoxyribose, hence the name deoxyribonucleic acid (DNA). RNA, on the other hand, is composed instead of just ribose, thus called ribonucleic acid (RNA). When a gene is transcribed, the genetic info, in the language of nucleotide bases, is carried out of the nucleus, into the cytoplasm, where it will arrive at a ribosome. Ribosomes are the protein factories of the cell. The ribosome is the station where the RNA language will be decoded, or "translated" into proteins, whose alphabet is based upon amino acids.
DNA remains inside the cell's nucleus, except for when the cell divides (mitosis). The DNA is like the blueprint for the cell's architecture; it must remain protected from dangerous chemicals or pathogens which may enter the cytoplasm. It's information, however, must be brought outside the nucleus in order for its messages to be carried out (as proteins). RNA becomes that bridge from message to function. RNA can transcend the nucleus, existing both inside and out. When a gene's message is transcribed from DNA to RNA, the message can then be transported to the final destination, ribosomes. Since it's carrying the message, this transcription RNA is termed messenger RNA (mRNA). It is made inside the nucleus and then exits through tiny nuclear pores, into the cytoplasm, where it will seek out a ribosome. mRNA is only single-stranded, unlike DNA, which is double-stranded. The mRNA is like a long ribbon of codes; DNA is doubled stranded, to protect its message and to have a backup. Another difference between the mRNA and DNA is that DNA contains the 4 bases: adenine, thymine, cytosine, and guanine. mRNA has 3 of them, but each thymine is replaced with the base uracil.
It can be said that the majority of the work of creating a protein from a gene of DNA is carried out by RNA. Mostly through the process of translation, 3 different types of RNA make this possible: mRNA, transfer RNA (tRNA), and ribosomal RNA (rRNA). Each has a different role in "translation," the process of manufacturing proteins (language of amino acids) from the language of nucleic acid. We've seen mRNA's purpose of carrying the genetic information out of the nucleus, and towards ribosomes. But once arrived, how exactly is genetic information translated into amino acids? Well that's where tRNA and rRNA come in. When an mRNA "docks" on a ribosome, it's the rRNA of the docking site of the ribosome that recognizes the start end of the mRNA strand. The ribosome reads 3 nucleotides at a time, and it's 3 nucleotides which translate into a single amino acid. Each 3 is termed a codon, and when each codon is "read" by the rRNA, it "requests" a transfer tRNA specific to that codon. Each tRNA carried just one specific amino acid, and the amino acid must be specific for the 3 nucleotide codon that it's tRNA translates into. In this fashion, 3 nucleotides at a time, the ribosome "calls in" the specific tRNA, with specified amino acid attached, that matches the codon that the rRNA reads. Then the ribosome attaches each new amino acid to the growing polypeptide chain. And that's the summary of how RNA is tantamount (absolutely necessary) to making a protein within a cell.
