Huck Finn and his friends and aquaintances were all fictional characters; the town of St. Petersburg, Missouri never existed.
However, many of the activities and situations portrayed in the story are very representative of events that occurred, and many of the settings of action are real. The Mississippi River exists; there were steamboats that cruised up and down it; it would have been possible for a runaway on a raft to float down the river, camping along the banks as needed.
Traveling entertainment in the form of circuses or entertainers did make their way throughout the country, printing and handing out advertising posters as they went. Slavery was a fact in the areas in which the story is set - Jim's concern about being detected and identified as a runaway slave would have been very real and justified.
While The Adventures of Huckleberry Finn would generally not be classified as a work of historical fiction, it certainly uses a very accurate historical background as the setting for its fictional story.