A declaration of conformity can be requested by the buyer for any good from the supplier - more often than not the manufacturer. It is one of 4 basic certification types listed in european standard EN 10204, and basically states that the goods meet the order as specified, and usually there's a note listing that they were made to specification, and will list any legal requirements it meets should they form part of the order (This is also known as a 2.1 cert - a 2.2 also lists the main materials used and test specifications met, a 3.1 lists the exact materials used and provides evidence of tests, and the 3.2 is the same as 3.1 but supplied by an independent test house such as Lloyd's or DNV)
A declaration of incorporation is different - it's for part-finished goods, and states that the goods cannot meet the final specification at the time they left the supplier's premises, but were made to be included in a larger job and are correctly made to that point, and that the components meet the ordered standard.
The sad thing is I know all this off the top of my head as it's how i fill about 20% of my working life...!