This changes the way the light hits the model and makes it so any higher poly image can be mapped onto the lower model creating the look of a higher poly and quality mesh without using more tris. This is mostly used when creating characters as they require a high amount of detail but are used on objects as well to optimize a game or whatever they are being used for.
Displacement maps create texture by creating the surfaces so that they can be edited. This means that you can edit the displacement map without affecting the original mesh. This means that extra detail can be added on top of the original mesh to either smooth out add depth to an already created model without having to add in a lot of extra tris.
Bump maps are used to simulate a ruff texture such as wrinkles or bumps on a surface. this is done by ‘perturbing the surface normals’ which means either changing or unsettling the normal’s of the object. This map essentially uses gray scale in order to create the illusion of having a texture.