What are artifact creatures?
As the name suggests, these cards are both Artifacts and Creatures. This means that anything that affects either artifacts or creatures affects them. Artifact creatures can be anything from robots, Animated objects, to living creatures infused with metal. Artifact creatures can not affect creatures of the same type or name unless printed otherwise.
Artifact creatures have always been present in Magic. In Alpha there were four artifact creatures. Most sets after that have had at least some artifact creatures. Mirrodin Block, a set that was all about artifacts, naturally had a lot more artifact creatures. More recently in the Shards of Alara set one of the planes, Esper is populated solely by artifact creatures.
There are certain abilities that Artifact Creatures have that no other creature has. These abilities are:
Modular X - This creature comes into play with X number of +1/+1 counters on it. When this creature is put into the graveyard from play, you may distribute all +1/+1 counters on this creature to another Artifact Creature.
Sunburst - This creature comes into play with a +1/+1 counter for each different color mana used to play this creature. Source: Classic sixth edition rulebook.
Generally, artifact creatures are better than other creatures due to the many combinations that you can make with them. Many cards combo with other artifact creatures.
Artifact and color
Most artifact creatures require no colored mana, meaning that you may choose the color of the mana. But some need a specific color to activate their effect. example: Nim Replica
But there are also Artifacts with require a mana is a certain color to be used. example Tidehollow Strix
Even though Most artifacts are Colorless, recent expansions have released Colorless Creatures and Instants as well as Enchantments and Tribal Spells, which were among Lorwyn´s additions to the game.