When adding soap why does it help remove dirt and oil?

Firstly, it is important to know the chemical structure of soap. Soap is made of a long hydrocarbon tail (R), which is hydrophobic (water fearing) and a polar, hydrophilic head, made of a COO- group connected to some cation (usually Na+). Now, for example, imagine a surface covered in oil placed in water. The oil will remain on the surface and not mix with the water. If a soap is added the hydrophobic hydrocarbon chain is attracted to the non-polar oil and bonds via dispersion forces, while the positive COO- head is attracted to the water, thus you have the tails in the oil and the heads in the water. Now, if the water is agitated the hydrophilic heads will want to move with the water, as the head of the molecule moves, so to does the tail, thus the tail moves into the water, still connected to the oil, removing the oil. A diagram such as this will help in understanding: http://en.wikipedia.org/wiki/File:Soapfunction1.jpg