Music theory is a vast topic which evolved through long period of time and still evolving. There are different varieties of musics practiced throughout the world and that makes the things more complex. Though the theory is complex yet there are many commonalities between them and that helps understanding the theory easily. Wikipedia is a good source of information.
I have found an Android app called Music Companion which helped me a lot in understanding different Scales, Chords to be played in any Scale, formation of Chords, Circle of Fifth(/Fourth) for all the scales etc. It also has a built in Metronome with very good timing and chords can be played along with the Metronome beats.
The app can be found here:
Hope this will help other music enthusiasts as well.