Generally speaking, when one person contracts with another, he anticipates that that other will perform the contract. Where the contract relies on the particular skill of the other person, that person cannot delegate the work: an artist if asked to paint a picture must paint it personally and paint the whole of it personally. In certain types of contract, however, delegation of all or part of the work must be anticipated, particularly in building contracts, where a great part of the work is subcontracted. Subcontracting should be distinguished from assignment of contract. The liability for breach of contract still remains with the original contracting party.
Reference: The Penguin Business Dictionary, 3rd edt.