If you see two of whatever you are looking at, you may have a condition known as double vision, also referred to as diplopia.
If you can see objects at a distance clearly but have trouble focusing well on objects close up, you may be farsighted. Farsightedness or long-sightedness is often referred to by its medical names, hypermetropia or hyperopia.
Poor vision that cannot be corrected fully with glasses may indicate a condition known as conical cornea or keratoconus. A rare condition, keratoconus primarily affects people in their early 20's.
If you can see objects nearby with no problem, but reading road signs or making out the writing on the board at school is more difficult, you may be near- or shortsighted.
Do you occasionally see specks or threadlike strands drifting across your field of vision? Then, when you try to look at them, do they seem to dart away? If so, you're seeing what eye care practitioners call spots or floaters