Illusion is a creature type.
All recent Illusions are Blue creatures with a high Power/Toughness for a relatively low Mana Cost. Flying is quite common amongst them because they are blue creatures. All recent Illusions have the effect: "When this card is the target of a spell or ability, Sacrifice it.""
2012 Core set introduces a lot of great Illusions and Lord of the Unreal can greatly boost them with +1/+1 and Hexproof. With Illusions you can get a 2/2 for 1 mana or a 5/5 Flying for 4 mana or pay only 2 mana to copy any creature your opponents or you have as an Illusion clone.