As awesome as this lesson is, we’ve only really scratched the surface of music notation. There are many excellent websites that go into more detail about other aspects of music notation, including symbols telling a performer how loud or quietly to play (dynamics), how to play the notes (articulation and expressions), or even when not to play (what, you think that all eighty or so members of an orchestra play non-stop from start to finish)? Check out these sites for further information about general music notation:
http://www.teoria.com/tutorials/index.php
http://www.musictheory.net/lessons
http://method-behind-the-music.com/theory/notation
Music is more than just dots on a page: a massive vocabulary has evolved over the centuries to describe all kinds of minutiae within the music. There are oodles of great online music dictionaries, but by far the most comprehensive I’ve found is the Virginia Tech Music Dictionary. It’s especially helpful because each entry has a little audio file telling you how to pronounce the word – very useful since most descriptors in music are in another language!
Feeling like a challenge? A big part of learning an instrument is training your ear to recognize the difference between notes. This is called aural or ear training. There’s many websites that aim to help young musicians get their ears into tip-top shape, but this is the one I've found to be most user-friendly.