Difficulties inherent in ASCII and Extended ASCII, and Unicode Compatibility?

What are the difficulties inherent in ASCII and Extended ASCII and how these difficulties are overcome by Unicode?

Can some one explain me the unicode compatibility?

And what does the terms associated with Unicode like Planes, Basic Multilingual Plane (BMP), Suplementary Multilingual Plane (SMP), Suplementary Ideographic Plane (SIP), Supplementary Special Plane (SSP) and Private Use Planes (PUP) means.

I have found all these words very confusing