Explanation:

There are four ways in which Indian citizenship can be acquired: birth, descent, registration and naturalisation. The provisions are listed under the Citizenship Act, 1955.
By Birth:
Every person born in India on or after 26.01.1950 but before 01.07.1987 is an Indian citizen irrespective of the nationality of his/her parents.
Every person born in India between 01.07.1987 and 02.12.2004 is a citizen of India given either of his/her parents is a citizen of the country at the time of his/her birth.
Every person born in India on or after 3.12.2004 is a citizen of the country given both his/her parents are Indians or at least one parent is a citizen and the other is not an illegal migrant at the time of birth.
By Registration: Citizenship can also be acquired by registration. Some of the mandatory rules are:
A person of Indian origin who has been a resident of India for 7 years before applying for registration.
A person of Indian origin who is a resident of any country outside undivided India.
A person who is married to an Indian citizen and is ordinarily resident for 7 years before applying for registration.
Minor children of persons who are citizens of India.
By Descent:
A person born outside India on or after January 26, 1950 is a citizen of India by descent if his/her father was a citizen of India by birth.
A person born outside India on or after December 10, 1992, but before December 3, 2004 if either of his/her parent was a citizen of India by birth.
If a person born outside India or or after December 3, 2004 has to acquire citizenship, his/her parents have to declare that the minor does not hold a passport of another country and his/her birth is registered at an Indian consulate within one year of birth.
By Naturalisation:
A person can acquire citizenship by naturalisation if he/she is ordinarily resident of India for 12 years (throughout 12 months preceding the date of application and 11 years in the aggregate) and fulfils all qualifications in the third schedule of the Citizenship Act.
The Act does not provide for dual citizenship or dual nationality. It only allows citizenship for a person listed under the provisions above ie: by birth, descent, registration or naturalisation.