Complex sentence: Turn of the century, in its broadest sense, refers to the transition from one century to another, although in both common and formal use, it usually specifically refers to the transition from the 19th century into the 20th century.
Split into simpler sentences: Turn of the century, in its broadest sense, refers to the transition from one century to another. Where no specific century is stated, the term usually refers to the transition from the 19th century into the 20th century.