The story of the "Seven Against Thebes" is about the war waged between the two sons of Oedipus: Eteocles, who led the Theban army, and his brother Polynices, who led an Argive army against Thebes. The "seven" were the captains in Polynices' army: Tydeus, Capaneus, Eteoclus, Hippomedon, Parthenopeus, Amphiaraus and Polynices himself. In the image below, you can see the seven of them swearing their oath: they have slaughtered a bull and have each dipped their hands into the blood of the bull; with that blood they swear that they will either capture the city of Thebes or die in the attempt. In the end, Eteocles and his army drive the captains away from the gates of Thebes, although Eteocles and Polynices both fall on the field of battle. This then leads to the tragic story of Antigone, who resolves to give her brother Polynices a symbolic burial, even though her uncle Creon has ordered that the body of Polynices should be left in the open as carrion.
For more about the story of the Seven, you can read this account by Alfred J. Church, based on the tragic drama by Aeschylus. You can also read E. D. A. Morshead's English translation of Aeschylus online. The epic poem by the Roman poet Statius, The Thebaid, provides the most detailed ancient account of the Seven.