All intervals are labeled by how far apart they are: c to g is a fifth c d e f g - 5 notes. Each interval has more than one way of being defined. A third can be a major or minor (c - e is major, c - eb is minor). A fifth can be perfect (c - g) or diminished (c - gb). Play them on your instrument to hear the difference in sound.
If you want to learn more about the different intervals, see me - I have some computer software to help you.