Tax on Turnover/Income/Profit ?

Its a very basic question,

1) When Government says 30% tax for Corporates, is it on Total Annual Turnover OR Total Annual Income OR Total Gross Profit ?

2) If the answer to above question "Profit", then why the government taxes the WHOLE income of an individual ?

3) Point  1  above  differ for different forms of company (Proprietership/Partnership/LLP/Pvt Ltdd/Public Ltd) ?

Thanks...............