All protists are eukaryotic (have a nucleus), most live in water. They are the "ancestor organisms" because they were the first organisms ever to exist.
Protists are simply organisms that don't fit into any of the other kingdoms.
The great diversity of form, habitat, mode of nutrition, and life history exhibited by eukaryotes suggests they evolved several times from various groups of prokaryotes. This makes the Protista a polyphyletic group. Eukaryotes are generally larger, have a variety of membrane-bound organelles, greater internal complexity than prokaryotic cells, and has a specialized method of cell division (meiosis) that is a prelude to true sexual reproduction. Protists might be viewed as a group from which the other eukaryotic kingdoms evolved.