What defines the group of organisms referred to as protist

Well protists quite very as organisms i.e it is hard to say if an organism is a protist. Some main features of protists are that they have 
1. A membrane enclosed nucleus
2.They are mainly single celled

Also protista are found mainly anywhere water is present for example damp soil, pools and lakes. 

Hope this helps :).

It changes food into energy that the organism can use.