What is the difference between semantic segmentation, object detection and instance segmentation?
Object Detection : is the technology that is related to computer vision and image processing. It's aim? detect objects in an image.
Semantic Segmentation : is a technique that detects , for each pixel , the object category it belongs to , all object categories ( labels ) must be known to the model.
Instance Segmentation : same as Semantic Segmentation, but dives a bit deeper, it identifies , for each pixel, the object instance it belongs to. The main difference is that differentiates two objects with the same labels in comparison to semantic segmentation.

Here's an example of the main difference.

In the second image where Semantic Segmentation is applied, the category ( chair ) is one of the outputs, all chairs are colored the same. In the third image, the Instance Segmentation, goes a step further and separates the instances ( the chairs ) from one another apart from identifying the category ( chair ) in the first step.
Hope this clears it up for you a bit.