Ribosomes are a type of organelle found in every cell, and their main function is to synthesize proteins for use throughout the cell. In the ribosomes, individual amino acids are arranged into long protein chains according to the sequence of mRNA, or messenger RNA. It is the ribosome's job to read the sequence of mRNA and translate it into the correct sequence of amino acids for each protein that must be made.
The process by which ribosomes make protein calls for several steps. First, the mRNA is made in the nucleus of the cell and sent to the ribosome. The mRNA then combines with the ribosome's subunits, which themselves are made of protein, and is interpreted into a new sequence known as tRNA, or transfer RNA. Each nucleic acid of the tRNA chain is bound to an amino acid. Once the tRNA chain is complete, the ribosome pulls the amino acids off of the tRNA and connects them into a long protein chain.
All prokaryotic and eukaryotic cells contain ribosomes, though those found in eukaryotic cells are larger. Some ribosomes, known as free ribosomes, are found directly in the cytosol. Others are bound to the cell's endoplasmic reticulum, which makes it easier to transport their finished products throughout the cell. Ribosomes are also found in mitochondria and chloroplasts, and they serve the same function -- to make proteins for these organelles to use.