The soap contains hydrophobic (i.e. fat soluble or water repelling) as well as hydrophilic (water soluble) end. The hydrophobic end remains soluble in dust or grease particles while the hydrophilic end is attracted towards the water. Hence, several hydrophobic molecules surround the grease particles. This aggregate of soap and dirt is called a micelle. On washing with plenty of water, dust is removed along with soap.