Note blocks do just what they say on the tin. When a block has a redstone pulse applied to it, it produces a single tone. Right-clicking on the note block will change the note, up to two different octaves.
The block that the note block is placed on top of affects the instrument sound that is produced.
- Wood-based blocks (including another note block)- Bass Guitar
- Sand / Gravel / Soul Sand - Snare Drum
- Glass / Glass Panes / Glowstone - Clicks / Sticks
- Stone-based blocks (e.g. ores) / Obsidian / Netherrack / Bricks - Bass Drum
- Dirt and all other blocks including Air - Piano / Harp
By using redstone currents and repeaters, it is possible to make a simple doorbell, to a alarm system to quite advanced reproductions of songs - there's quite a lot on YouTube but here's one of the first I saw.
You can see two "tracks" - one provides the main melody and the other is the bass line.
If you search for your favorite song and "minecraft note blocks" you'll be impressed with what you can find.