The Comptroller and Auditor General of India is the Constitutional and Independent Authority in India, established under Article 148 of the Constitution of India.
He is the head of the Indian audit as they are authorized to audit all receipts and expenditure of government of India and state governments,including those of autonomous bodies and corporations substantially financed by the Government. 
They are entitled to audit the Consolidated fund of India, Contengency fund of India and public account so it is said to be chief guardian of public purse.

CAG is appointed by the President by warrant under his  hand and seal and provided with tenure of 6 years or 65 years of age,  whichever is earlier. They can’t be appointed twice.
CAG can be removed by  the President only in accordance with the procedure mentioned in the Constitution that is the manner same as removal of a Supreme Court Judge.
He is ineligible to hold any office, either under the Government of India  or of any state,once he retires/resigns as a CAG.

As per the provisions of the constitution, the CAG’s (DPC) (Duties, Powers and Conditions of Service) Act, 1971 was enacted. As per the various provisions, the duties of the CAG include the audit of:
• Receipts and expenditure from the Consolidated Fund of India and of the State and Union Territory having legislative assembly.
• Trading, manufacturing, profit and loss accounts and balance sheets, and other subsidiary accounts kept in any Government department; Accounts of stores and stock kept in Government offices or departments.
• Government companies as per the provisions of the Companies Act, 2009 Corporations established by or under laws made by Parliament in accordance with the provisions of the respective legislation.
• Authorities and bodies substantially financed from the Consolidated Funds of the Union and State Governments. Anybody or authority even though not substantially financed from the Consolidated Fund, the audit of which may be entrusted to the CAG.
• Grants and loans given by Government to bodies and authorities for specific purposes.
• Entrusted audits e.g. those of Panchayati Raj Institutions and Urban Local Bodies under Technical Guidance & Support (TGS)

.Limitations of CAG :
• The CAG officials only see accounts that government departments want them to see.
• The CAG cannot call for particulars of expenditure incurred by the executive agencies but has to accept a certificate from the competent administrative authority that the expenditure has been so incurred under his authority.
• The CAG officials are often under pressure of time and the concerned officers says the related files are lost.

The solution to the problem is to digitize the receipts and expenditure to be audited, make government accountable for transparency and CAG get as much time as needed to make adequate audit report.

Sources : The Hindu, Wikipedia

“I am of the opinion that this dignitary or officer is probably the most important officer in the Constitution of India. He is the one man who is going to see that the expenses voted by Parliament are not exceeded, or varied from what has been laid down by Parliament in the Appropriation Act.” — Dr. B.R Ambedkar