The nucleus is found in eukaryotic organisms, while nucleoids are found only in prokaryotic organisms. 

Eukaryotic cells have a large, well-organized, spherical nucleus that takes up a large portion of the space inside the cell. The nucleus contains the genetic material of the organism and is protected by a double membrane called the nuclear membrane, which separates the nucleus from other cellular components of the eukaryotic cell.  

Prokaryotic cells have a small, unorganized nucleoid region that does not have a distinct shape and takes up only a small portion of the space inside the cell. The nucleoid contains the genetic material of the organism but it lacks a protective membrane and is not separated from the other cellular components of the prokaryotic cell.