Both -logy and -logist are Greek and Latin suffixes.
-logy means the science or study of; indicating the writings or discourse of
-logist refers to someone who practices and is an expert in fields of -logy
If you've ever seen any of the Indiana Jones movies, Indiana specializes in archaeology; thus, he is an archaeologist.
The medical profession is filled with examples. A doctor who studied dermatology is a dermatogist. A physician who knows cardiology is a cardiologist. A doctor schooled in gastroentrology is a gastroentrologist. A doctor who knows neurology is a neurologist. The poor medical student who studied proctology is a protologist.
There are less dark professions. A person who studies musicology is a musicologist, presumably happy.
Here are some more examples:
A biologist studies biology.
An anthropologist studies anthropology.
A geologist studies geology.
A psychologist studies psychology.
A sociologist studies sociology.
An immunologist studies immunology.
Smokers often need a pulmonologist who specializes in pulmonology, the study of the lungs.
As you can see, knowing Greek and Latin roots and affixes is a great way to build your vocabulary.