What is the current system of individual taxation? What is a flat-tax system?

Some opponents say: there are too many loopholes which can help wealthy people reduce their taxation in the current individual income tax system. I wonder what are those loopholes?Can you explain as detailed as possible?

What are the differences between a flat-tax system and the current system of individual taxation? Can you compare these two systems and give me some examples and details of those differences?

What are the advantages of flat-tax system and what are the disadvantages of the current individual income tax system?

Why current individual income tax should change and how it has changed already?

Who is benefitting from each system?

Should current individual income tax system be replaced by flat-tax system?