Reading music from the printed sheet music involves several inter-related areas including melody, harmony, and rhythm. Within those areas are key signatures, leger lines, time values, time signatures, note values, chord structures and progressions, rhythm patterns and several other related things such as dynamics, touch, sight-reading and so on. Watch this very short video to get a feel of the overall process of reading music:

For a course on music reading for beginners, please go to How To Read Music Fast!