Servos are more complicated than DC motors and are more suitable for a situations in which you need more control over the exact position of something attached to the servo. A servo will usually contain a DC motor, gears, control circuits, and a sensor to detect the exact position of the servo. All these components come together in a device that lets you have more precise control over the exact angle that the servo is pointing at.
The following image shows a disassembled servo in which you can see the DC motor, gears, and circuitry:
Source: https://commons.wikimedia.org/wiki/File:Exploded_Servo.jpg
Servos work great in applications in which you need exact control over the angle of a certain part; for example, where you need to control the angle of a robotic arm or the angle of a rudder on a boat.