Sibelius is a well-known tool for music composers, teachers and enthusiasts, it's a package that contains the means for making a music score, playing it in preview, sharing it online and edit your creation in any way you see fit.
The interface provides you with quick access to score writing and offers many useful features such as editing of scores, printing them, applying any of the music notations included into the program's database. This makes it possible to create and refine the most complex pieces of music on the professional level. You then have the option to share the composition you wrote in Sibelius online using the program's Internet capabilities.
Other notable features include the ability to play the music in MIDI in preview mode or transfer compositions to different audio files in order to transfer them to a physical drive. The program offers many samples to shape your music the way you want.
Due to its sharing capabilities, Sibelius is good for educational purposes. Music teachers can use it in school networks, since it allows sharing files and quickly see and hear the results. It can manage multiple copies of the project and helps control and edit compositions cooperatively, look at examples, etc.
Sibelius may well be the ultimate composing tool. At least, if you can afford it.