Assume you’re making a square picture frame. For the four sides to fit together, you need to cut each length end at an angle of 45 degrees. That’s a miter cut.
However, if you wanted this frame to have angled sides — leading in towards the picture — that’s a bevel cut.
Simply, in the case of miter saws, if the cutting blade is perfectly vertical, it’s a miter. If it’s angled, it’s a bevel.