Sliver is a creature type.
They are a creature type unlike any other in Magic: The Gathering. While a few other instances of a card with a specific Creature Type, such as Warriors and Soldiers, share abilities with other creatures of the same type in play, every single Sliver has some effect on others of the type. Slivers were also the first to be designed this way, and are still, as of 2015, still the most numerous of this design in total creatures, and the most powerful in effect.
When slivers were first introduced to the game they dominated competitive play. Their ability to win games caused Wizards of the Coast to not include them in other sets. The slivers did return however, but they do not compare to their former glory. There are slivers of every color.
In fact, the first card of every color was the Sliver Queen, the first of three "sliver gods". Each of the sliver gods has a mana cost of one of each color, they are all 7/7's, and they all have incredible effects. Sliver Queen allows for a player to generate an army of 1/1 sliver tokens. Sliver Overlord lets a player find any sliver he or she need from their library. Sliver Legion gives all slivers +1/+1 for each other sliver in play.