Object detectors find objects in an image by computing regions of interest (ROI), then draw a bounding box around it, and finally, run a classification on the ROI to predict the object's class.
The output of an object detector is a set of bounding boxes including its (x,y) coordinates, width, height, and the assigned class with a confidence score. Its performance is usually measured with the mAP.
In comparison to semantic segmentors, object detectors can identify specific objects and count the occurrences in an image. However, unless you're labeling rectangular objects, the bounding boxes will contain some noise influencing the classifier's performance negatively—instance segmentation yields better results often. In comparison to instance segmentors, object detectors require less computational resources and less labeling effort, though.