Desu is desu. It is d, it is e, it is s, it is u. It is known by many other names, such as da or deshou or datta. But these are just all but a clever ruse. Behind all these is desu, the one, the only, the eternal.
To put it simply, my child, desu is far too complicated for you to truly grasp and understand.
Oh and mind you, desu also has a bastard Korean brother, nida. Watch out for nida, that good for nothing son of a bitch...