The full name of ERCP is endoscopic retrograde cholangio pancreaticography which is a way of looking at the drainage of the liver and the pancreas for any abnormalities. It is usually used when gall stones are impacted in the biliary tree but can be used for other situations as well. It is similar to a gastroscopy but more technically difficult and involves radiation. This is a diagnostic and therapeutic procedure.
MRCP, is an MRI technique without endoscopy, which allows for very nice pictures of liver and pancreatic ducts. This is a diagnostic procedure and sometimes results in the need for ERCP at a later time.