The Great Fish was a huge fish that saved Jonah the prophet from drowning by swallowing him. Jonah stayed in the creature for three days and three nights until it finally spat him out onto the shore.
The people of Israel and Judah did not like the sea. There promised land was instead along land routes between the superpowers of the day. Among the most hated of the so-called gods of their day was the Phoenician god "Dagon" worshipped by the coastal nation of Philistia. It was to this god's temple that the Philistines brought the Ark of the Covenant when they captured it. The idol there did not fare well in the presence of God's "footstool."
Nevertheless, when Jonah was called on to go to Nineveh on the Tigris, he ran in the opposite direction, booking passage on a ship heading into the Great Sea. When God wanted to get Noah back, He "prepared" a fish (a "dagon") to swallow his erring ambassador to the heathen megalopolis. When Jonah convinced the sailors that tossing him overboard would calm the storm -- as it did -- he most likely expected to die very soon. However, his life was preserved, though it literally "felt like hell" (or to him, "Sheol").
When the fish had had enough of the prophet, he spit him out. Mercifully, this was on to the shore up towards Nineveh. This, too, was an act of God, for the LORD told him when and where to do it.
The fish was never identified as to species, though men have both ridiculed its story and labored to locate a creature who could duplicate what it had done. With the numerous extinct creatures, and still undiscovered sea creatures, speculations are fruitless. The truth is, this fish was a miracle from God, for the purpose of both an immediate mission and a lasting type of His work among men.
Soon after the fish swam back into the deep sea, Jonah was called once again to go to Nineveh. The result was astounding, with one of the largest city-states of the day being turned to faith in the true God within a deadline which the prophet thought would end in its destruction.
More importantly, the miracle served as a type of the resurrection of Jesus Christ. Jesus himself would use it in predicting this world-changing event. The time that Jonah had spent inside the fish (which he called the "belly of Sheol (Hell, or the grave)" served as a prediction to the time Jesus would spend in the "belly of the earth" (that is, the grave)