The genre predates the mainstream popularity of science fiction proper, and does not necessarily feature any scientific rigor, being instead romantic tales of high adventure. For example, little thought is given to explaining why the environment of the alien planet is compatible with life from Earth, just that it does in order to allow the hero to move about and interact with the natives. Native technology will often break the known laws of physics.
The genre tag "sword and planet" is constructed to mimic the terms sword and sorcery and sword and sandal. The phrase appears to have first been coined in the 1960s by Donald A. Wollheim, editor of Ace Books, and later of DAW Books at a time when the genre was undergoing a revival. Both Ace Books and DAW Books were instrumental in bringing much of the earlier pulp sword and planet stories back into print, as well as publishing a great deal of new, imitative work by a new generation of authors.
There is a fair amount of overlap between sword and planet and planetary romance although some works are considered to belong to one and not the other. Influenced by the likes of A Princess of Mars yet more modern and technologically savvy, sword and planet more directly imitates the conventions established by Burroughs in the Mars series. That is to say that the hero is alone as the only human being from Earth, swords are the weapon of choice, and while the alien planet has some advanced technology, it is used only in limited applications to advance the plot or increase the grandeur of the setting. In general the alien planet will seem to be more medieval and primitive than Earth. This leads to anachronistic situations such as flying ships held aloft by anti-gravity technology, while ground travel is done by riding domesticated native animals.
In A Princess of Mars, John Carter, a Confederate officer and soldier, has taken up prospecting in Arizona after the war to regain his fortune. Under mysterious circumstances, he is transported to Mars, called Barsoom by its inhabitants. There he encounters savage and monstrous aliens, a beautiful princess, and a life of adventure and wonder. Burroughs followed up this first book with several more Barsoom stories, and another series that could be considered Sword & Planet, featuring as hero Carson Napier and his adventures on Venus, natively known as Amtor. Burroughs' Pellucidar series could arguably be considered sword-and-(inner) planet, as it follows most of the plot conventions described below.