What is deference between EPC contract and Turn Key contract?

Essentially an EPC project is the same as a turnkey project. An EPC contractencompasses the E (Engineering/Design), P (Procurement) and the C (Construction) which is the entire project from concept to commissioning (turnkey). However EPC is a very broadly and sometimes wrongly used term.

An EPC Contract encompasses the E (Engineering/Design), P (Procurement) and the C (Construction)  and pre-commissioning & commissioning of individual systems involved in the project. However a Turnkey Contract which is the entire project from concept to commissioning and startup of the entire plant/system up to stabilize the operation and end product.

In general an EPC contract the responsibility limit up to Detailed Engineering, Procurement & construction and a Turnkey Contract the responsibility is limited up to the end product.