Thor was an Asgardian god, a mythical character based upon ancient Norse heroic legends. Here he was treated as a superhero in a high-fantasy setting complete with a background and cast of characters that supported his mythological roots. Thor's adventures usually centered around his homeland of Asgard and the other gods and demi-gods that populated it; one important foe appeared early on in the form of Loki, Thor's wicked half-brother, who was always hatching some scheme to kill his hated sibling.
The title evolved as time went on; it started out very strong with Jack Kirby delivering some of the best and most dynamic artwork of his career - characters arrayed in armor taking part in sprawling battle scenes, or Thor and some equally-muscled counterpart locked in Olympian combat. Stan Lee's writing was up to the task as well: he seemed to revel in the sweep of the narrative, especially the faux-Old English speech of the characters.
After Kirby left, John Buscema took over the art chores; and while Buscema's work was as perfect as ever, unfortunately the writing just couldn't make readers care too much about what was going on in the title. There was a revival of creativity in the book in the late 70's, particularly in the storylines leading up to issue #300, wherein readers finally learned the true nature of all things Asgardian - i.e., how Odin had sworn to protect Earth from the Celestials, the origin of the Odinsword, etc. It was all a phantasmagorical masterpiece that summed up everything that was good about the title and its fictional world.
The 1980's would introduce a 'new' Thor in the form of Beta Ray Bill, a horse-faced alien who was deemed worthy enough to take on the mystical hammer and the godly mantle assigned to it. Artist Walt Simonson was responsible for the pencilling during this run, and his work (sketchy though it was) was well-received by fans.