Short answers, pulled from the story.
Orestes was the son of Agamemnon and Clytemnestra, and the brother of Electra and Iphigenia. He killed his mother Clytemnestra to avenge his father's murder, an act commanded by Apollo that brought the Erinyes in pursuit. A formal trial on the Acropolis of Athens, with a tied vote decided by the goddess Athena, ended with his acquittal.
Orestes killed Clytemnestra to avenge his father Agamemnon, who had been murdered by Clytemnestra and her lover Aegisthus. Apollo had commanded the act, a defense Orestes cited at his trial in Athens.
In Aeschylus's Oresteia, Orestes kills his mother to avenge his father and is then pursued by the Erinyes. Athena convenes a formal trial on the Acropolis of Athens with twelve judges; when the vote ties, she casts the deciding ballot for acquittal, and the Erinyes are transformed into the Eumenides, the Venerable Goddesses.
In Euripides' play, Apollo orders Orestes to travel to Tauris to retrieve a statue of Artemis that had fallen from the heavens. He arrives with Pylades and both are captured; the local priestess assigned to sacrifice them turns out to be his sister Iphigenia. The three recognize each other and escape together, carrying the statue.
Pylades was the son of King Strophius and of Anaxibia, the sister of Agamemnon, making him a first cousin of Orestes. He accompanied Orestes throughout his exile and on the mission to Tauris. A Roman-era dialogue attributed to Lucian, entitled Erotes, cited Orestes and Pylades as the principal examples of homoerotic friendship.
Orestes died of a snakebite in Arcadia. Herodotus records that the Oracle at Delphi required the Spartans to recover his bones before they could defeat the Tegeans; a man named Lichas found them measuring seven cubits, roughly 311 centimeters. His ashes were later counted by the grammarian Maurus Servius Honoratus among the seven pignora imperii, the sacred pledges of the Roman Empire, kept at the Temple of Jupiter Optimus Maximus on the Capitolium.