A moving zigzag means a song is being imported. A green checkmark means iTunes has finished importing the song.
An exclamation point means the song file can't be located. It may have been moved or deleted.
A globe means the song is stored on the Internet, and "streamed" to you (via the Internet) when you click to play it.
A black checkmark means the song will be included in your next action. For example, only songs with black checkmarks are imported.
A speaker means the song is currently playing. When importing a song you're also listening to, you see a moving zigzag instead of the speaker.
When you eject a CD, the symbols disappear. For more status information, watch the display at the top of the iTunes window.