That's a very big question that can't really be answered simply here in this forum I believe,
DECS is used in IBM Lotus Notes Client
LEI is used in IBM Lotus Domino Server
You'll probably have to go to here http://www-142.ibm.com/software/sw-lotus/products/product4.nsf/wdocs/enterpriseintegrator?OpenDocument
and here http://support.lotus.com
and searh for LEI and select the Learning Option to start with.