Detection or localization is a task that finds an object in an image and localizes the object with a bounding box. This task has many applications, such as finding pedestrians and signboards for self-driving vehicles. The following image is an illustration of detection:
Segmentation is the task of doing pixel-wise classification. This gives a fine separation of objects. It is useful for processing medical images and satellite imagery. More examples and explanations can be found in Chapter 23, Object Detection and Chapter 24, Semantic Segmentation.