it could just be that you have scaled something with a minus value then frozen it's transformation, or extruded something against the normal direction, or something else - there are lots of ways this can happen. It's fine as long as you are aware that it has happened, and correct it. If I have made a model, I usually uncheck "double sided" under render stats. That way faces are only drawn in the viewport if their normals are towards the camera (so when you rotate the camera around the model, you'll see "holes" indicating back faces)
Reverse just reverses the normals on selected faces. Conform will try and make all selected normals face in the same direction. Sometimes it works better than others.