It depends on the context. For the particular case of force vectors acting on an object, the angle is usually specified relative to the surface it's acting on. This can either be specified from the horizontal (where 0 degrees is directly parallel to the surface and 90 degrees is directly perpendicular to the surface) or from the normal (where 0 degrees is directly perpendicular to the surface and 90 degrees is directly parallel to the surface). Though both are valid conventions, it's more common, in most textbooks I've encountered, for the angle to be specified relative to the normal. However, this may not necessarily be the case for the particular problem you're working on, so it might be a good idea to ask the assigner of the work what convention they're using (or, if you're reading from a textbook, to look towards the beginning, where such conventions are usually specified).