A thinker is just what it sounds like — a person who does a lot of thinking. If you're a thinker, it may take you a while to make an important decision.
You can use the noun thinker when you talk about a smart, scholarly person who's known for being an intellectual, or to describe someone who contemplates every choice at great length. Serious students are thinkers, and so are little kids who mull things over carefully, such as deciding which kind of doughnut to choose. Perhaps the most famous thinker is Auguste Rodin's bronze statue "The Thinker," which shows a man sitting, chin in hand, deep in thought.