All protists share one fundamental characteristic: they are eukaryotic organisms. This means that their cells contain a nucleus and other membrane-bound organelles. Unlike prokaryotic organisms, such as bacteria, which lack a defined nucleus, protists have more complex cellular structures.
This eukaryotic nature allows protists to perform various functions that are crucial for their survival. For example, the presence of a nucleus means that genetic material is organized and can be more efficiently expressed. Protists exhibit a great diversity in form and function, ranging from unicellular organisms like amoebae and paramecia to multicellular forms like algae.
In summary, while protists may differ greatly in their appearance and lifestyles, the commonality of being eukaryotic forms the basis of their classification in the biological world.