Retrieve retrieves a single record given the record ID, the method requires 3 parameters: the entity logicalname, the record ID and the columns (attributes) you want to retrieve. It throws an exception if the record ID is not found.RetrieveMultiple runs a Query against CRM, normally a QueryExpression is used that defines the entity logicalname, the conditions of the query and the columns (attributes) you want to retrieve. It returns always an EntityCollection object and the Entities property contains the list of the records that satisfy the query conditions, so you can have 0, 1 or n records returned.