Surface grinders' main property is an abrasive surface used for smoothening areas. Electrically powered grinders, such as belt sanders, can smoothen extremely rough pieces of metal or wood. Hand-held sandpaper can smoothen small bumps on wood in order to make it level.
Surface grinders can shape material -- but slowly. They are not like cutters that can slice through material and take off large chunks at a time. Instead, you can grind the material down to shape the ends of it into a curve, for example.
Surface grinders that do not require torque such as sand paper are used for finishing wood. Sandpaper has many different variations that include different grit size and grit type. Certain types of sandpaper abrasives such as garnet have wood sealing properties that close off the grain and make applying wood stains and varnishes a much easier and even task.
An angle grinder is used to angle metal. This is useful when you're preparing metal to be welded, or when sharpening blades. For angling wood, use a belt sander. Some belt sanders have a mechanism that allows you to angle them and lock into place. You also can hold the wood at an angle and sand it down.