Use of On as preposition 

Preposition on is used to explain something situated attached to something where one is attached under and another is attached upper. 

The pen is on table. 

Before, any book topics on

I read the book on magic. 

Here, On is used instead of about. 

The book about magic

Use of inspite of

 In spite of and despite uses as a preposition to denote 

Any worked have been done though there have sufficient level of restrictions to do it. 

In spite of my illness, I went to school.

Despite my illness,  I went to school. 

Regardless of uses when something is happen without consideration of any drawback of it. 

He got the job, regardless of his poor education .

In spite of/despite are same meanings. Both stands for doing something where something exist to restrict to do this.