A lumberjack is someone who cuts down trees professionally. Picture someone holding a chain saw and wearing a plaid flannel shirt and steel-toe boots, and you can picture a lumberjack.
These days, lumberjack sounds pretty old-fashioned — the same job is done today, but people who do it are typically called "loggers," and they use power tools like chain saws. Lumberjacks, particularly in the 19th and early 20th century, used hand saws and axes and were famous for being ruggedly independent and physically daring. Lumberjack was coined by Canadians.