CE Marking is a certification mark that indicates conformity with health, safety, and environmental protection standards for products sold within the European Economic Area (EEA).
If you have a product and you want to seel it you need a CE marking to be sold in the EEA. First of all you have to identify the EU requirements for your product and check if it meets them.
At this point you have three possibilities in front of you:
FIRST CASE: exist an harmonised standards for your product. The product can enter the market only with a CE Marking. In this case the producer has to turn to a Notified Body;
SECOND CASE: does not exist any harmonised standard for your product. The producer (not the Notified Body) will have to produce a Declaration of Performance. This step is not obligatory but can be necessary to respond the requests of clients that need a proof of performance. The tests can be developed by a third laboratory, unrelated to producer;
THIRD CASE: does not exist any harmonised standard for your product. The producer can request an European technical evaluation for the product and then obtain a CE Marking. In this case the producer should turn to a TAB (Technical Assessment Bodies). Very long and expensive process.