Believed to be the son of Shiv and Paarvatee Jee. Younger brother of Kaarttikeya. Elephant-headed god of Hindu. He is worshipped first among all Devtaa. He has 108 names, some of them are - Gajaanan, Ganapati, Vaachaspati, Vaageesh, Vighnesh, Vinaayak.

Who is Mahaa Ganapati?
Ganapati - Ganapati is combined word - Gan + Pati.
Gana means group / category etc    and    Pati means leader/lord. Because he is the Lord of Shiv's Gan that is why he is called Ganapati.

To be more detailed
Ga = intellect,    and   na = the higher knowledge/wisdom. He is the lord/leader of group, intellect, higher wisdom. As per the Hindu doctrines, before conducting any customs, it is a practice to pray Ganapati (so as to conduct the custom without any break or disturbance) . A form of Ganapati is Mahaa Ganapati. There are 16 forms of Ganapati, each form having its own significance.

He has eight Aavaran (surrounding) Deities - Ganesh, Gana-Naayak, Gana-Naath, Gan-Krd, Ekdant, Rakt-tund, Lambodar, Gajaanan, Mohodar, Vikat, Dhoomraabh, and Vighn-naashan. They all are different forms of Ganapati who is called by these names himself.