Planeswalkers are beings of energy, thought, and emotion all mixed into one. Most of the time we take the form we had before becoming planeswalkers. Human, elf, whatever. People who are born with a planeswalker "spark" are the only ones who can become planeswalkers. This happens through a particularily violent death. If someone with a spark dies of old age they simply die. If their spark ignites they shed their physical form and are given new life as a planeswalker.
Planeswalkers can travel to different universes or "planes", hence the name. All planes are part of a larget whole known as the multiverse, which is sentient even though it's made up of mostly empty space. In between all planes is an emptyness which is known by many names such as "the bastard plane" or "the blind eternities." For a nonplaneswalker being to enter this place means death. Not just any death, really really painful death. The person (or any other living thing) will both explode and implode at the same time. Then will be completely transformed into energy and scattered throught the multiverse. Not a very fun way to go. However living things may enter this place if they are protected, which is usually done by certain types of shielding spells.
To learn more about planes and planeswalkers read the Artifacts, Ice Age, and Invasion cycles in the Magic: The Gathering series of books.
