Spirit Monsters, originally as a rule, could not be Special Summoned, and are sent back to the owner's hand during the End Phase of the turn they are Normal Summoned, Flip Summoned, or flipped face-up. This also happens if they wound up in the opponent's hand via effects like Exchange. However, new Spirits have been released in The Duelist Genesis, one of which can be Special Summoned for a cost.
Many of the Spirit Monsters' designs were based on icons from Japanese mythology and they all have very similar backgrounds, supposedly of the spirit world.
Spirit monsters can only remain on the field temporarily (unless Set face-down), and thus their controllers run into a major problem of maintaining field presence, but many Spirit monsters have devastating special effects to make up for this deficiency. Two of the strongest Spirit monsters focus on maintaining a significant hand advantage over the opponent, and when combined withor other effects like that allow them to remain on the field, they can easily turn the tide of any duel in their favor. Though it wasn't an option in the past, there is currently enough Spirit support for decks entirely themed around them to be viable. Because Spirit monsters return to the player's hand, the play runs the risk of having to many cards during the end phase, forcing a discard, so is an effective way to counter this problem.