The origin of the word Maratha is in the ancient Marahatti language word `Marhatta'. That is a geographical term, which was used for all the people of Maharashtra. But later it was being used for all the warrior people of Maharashtra.
Chhatrapati Shivaji Maharaj gave an opportunity to fight with enemies to the people of all the castes in Maharshtra. For him, everybody was a Maratha, who helped for the freedom. So for him all his soldiers, including his Muslim soldiers (Who were in large numbers in his navy and in artillery) were Marathas.
It should be noted that Peshavas, who were Chitpavan Bramhins were also known as Maratha. The Muslim Queen of Ahmednagar Chandbibi was also known as a Maratha.
Later, Bramhins stopped to call themselves as Maratha.Today in Maharashtra a specific community who is politically strong, calls themselves as Maratha. This community has 96 clans. Most of the clans are newer (Did not existed before Shivaji Maharaj). But there are more than 30 clans which have ancient or medieval history. Some of such clans are:
The cultivaters of Maharashtra were known as Kunabis most of whom started themselves call as Marathas before one hundred years. This increased the number of Marathas rapidly in Maharshtra. So they became politically stronger. But the Marthas are divided in several groups namely Deshmukhs, 96 Kulis, Common Marathas, Kunabis and Kadus. They do not intermarry. There are geographical divisions also and they generally do not marry outside their region.(Like a Maratha of Sangli district marries with the maratha of surrounding districts of Satara and Kolhapur but not with the Marathas of Pune or Nagar districts. Which is just 110 Km. from Satara).
It is said that when a Kunabi becomes stronger, he becomes a Maratha, When a Maratha becomes stronger he becomes a Patil, When a Patil becomes stronger he becomes Deshmukh. (My addition: and when a Deshmukh becomes stronger, he becomes a Chief Minister!)
Deshmukh was the highest Designation, like District Collector in this era, while Patil is the chief of a village.
Generally, Marathas are not interested in the history before Chhatrapati Shivaji Maharaj. So, for them history starts with Shahaji Maharaj and ends with Sambhaji Maharaj. This wrong tendacy of Marathas was a plus poing for Vedic Brahmins, but now many Maratha scholars are finding their roots in ancient era, especially in Indus Valley Civilization. This is a good sign. Thanks to the great scholars like Dr. A.H. Salunkhe and others, who have written lot of books on the ancient history of Marathas.