In biology, the nucleolus is, strictly speaking, a "suborganelle" of the cell nucleus, which is an organelle.
It is a consequence of ribosomal RNA (rRNA) synthesis: nucleolar organizers , special regions on some chromosomes that contain multiple copies of the genes encoding for rRNA (which is involved in protein biosynthesis), gather themselves in the same region where they transcribe the rRNA genes. Thus it can be said the nucleolus consists basically of nucleolar organizers and the transcribed rRNA (plus associated proteins).
Following synthesis, rRNA molecules are attached to proteins, forming ribosomal subunits, which leave for the cytosol through nuclear pores.
Function: it creates ribosomes.