Hindus believe that Lord Vishnu has many important Avatars. Parashurama, also known as Parashuram Avatar, was the sixth important avatar of Lord Vishnu. In this avatar, Lord Vishnu was in the form of a human. He came to Earth to stop Chakravarti Samrat Karthaveerya Arjuna for he had become evil due to his ego and non humility, and as he was very powerful and had the boon of being the embodiment of the Sudarshan Chakra, the gods weren't able to defeat him and all the kshatriyas had become evil under him. Parashurama was the son of sage Jamadagni, one of the Saptarishis (or Saptarshis) and a sage who was known for his temper. It is believed that Parashurama was Chiranjeevi, meaning he was immortal. He was a great devotee of Lord Shiva. Parshurama's mother, Renuka Devi was born of a Kshatriya but she became a Brahmin afterwards (this was before the castes or varna were decided by birth).
Parashurama was born as Ramabhadra of the Bhargava Clan (headed by Bhrigu, one of the Manvantaras of Lord Brahma). He later gained the name of Parashurama when he obtained Mahakaal's Parashu (axe weapon) and lead the Devas to victory against the Asuras in a war in which the Devas were losing.
Parashurama was the first Brahmakshatriya (both a Brahmin sage and a Kshatriya warrior) and probably the only one. He could cleave his mind into two parts, one who would fight in battle and other would conduct a yagna in the shared mind.
He also played a vital role in Mahabharata, teaching Bheeshma, Dronacharya and Karna the art of war and weaponry and fought against Bheeshma to fulfil Amba's request to kill Bheeshma, though he failed to do so.
Parashuram also had a disciple, Akritavrana (literally meaning "no injuries"). He also contained an amsha (part) of Mahavishnu in the form of a Agya Chakra on his forehead between his eyebrows.
One of the interesting things about Parashurama was that, unlike the other Avatars of Lord Mahavishnu, he was not born an Avatar but instead, became an Avatar after he was born. This happened when he sparked the Amsha of Lord Mahavishnu inside him. He also had the blessings of the gods after he helped them win the Deva-Asura war, the blessings of Devi Chinnamasta, Bhadrakali, Shiva, Vishnu, Avadhuta Dattatreya and so on.
He was a master of weapons and especially the Parashu and the bow he wields. His conch's name is Amritajit. Since he is immortal, he is believed to teach the art of weaponry and war to Kalki, the last Avatar of Lord Mahavishnu in this cycle who will end this cycle, ushering in a new era.