Answer:
Both a human brain and a CPU perform computations.
A CPU is basically a serial device (a Von Neumann machine). Its computation steps are discrete. Its operations are chosen from a small set. It does some things, like arithmetic, extremely well. It never gets tired. It uses lots of energy. It interfaces with external devices (memory, ALU, Input and Output).
A human brain is a parallel interconnected device. Its computation steps are continuous. It has a large set of operations. It does some things, like face recognition, extremely well. It often gets tired. It is much more efficient energetically than a CPU. It interfaces with external devices (eyes, ears, nose, mouth, skin).
This is not even scratching the surface - Go read, say, "Goedel, Escher, Bach" in order to get a feel for how large this question is.