Original: They are characterised by the slipper-shaped pouches (modified labellums) of the flowers - the pouch traps insects so they are forced to climb up past the staminode, behind which they collect or deposit pollinia, thus fertilising the flower.
Simplified: They are special because of the slipper-shaped orchid pouches of the flowers. The pouch traps insects so that they must climb up past the staminode, where they get or give pollen, fertilizing the flower.