CEN - The European Committee for Standardisation. They write the EN Standards, to which products must conform if they're sold for a particular purpose in the EU.

CE - A CE mark means the item bearing the mark conforms to the relevant EN standard. It will be followed by a number which the identifying number of the relevant notified body that undertook the testing.

EN - These are the standards to which the item has been tested. They will define the testing methodology and the criteria required to pass as well as other things such as marking etc. Unfortunately, these are not freely available to the public but can be bought from the British Standards Institute (BSI). These standards are written by the CEN/TC 136/WG 5 working group which is a mix of national delegates, European manufacturers and notified bodies/testing houses(2).

When looking at the label attached to your kit it will have the CE Symbol, the number of the notified body and the relevant EN standard.

UIAA - Set by the International Climbing and Mountaineering Federation, UIAA ratings are often regarded as a gold standard of climbing safety equipment. Created before normalised European standards, they were the front runners in creating the original standards for climbing equipment. These standards are created by the Safety Commission. This is a group with delegates from climbing associations from all over the world as well as global manufactures and notified bodies/testing houses. They are a global standard, often slightly more onerous than the EN standards but are a voluntary standard that is not enforceable by law, unlike the EN standard within the EU. The lack of a UIAA mark does not per se mean that a product is unfit for purpose
UKClimbing:

When choosing to buy a product the average customer is unlikely to know how to assess whether any associated labelling is genuine, or if an unscrupulous manufacturer is lying – which does happen, for example, with some goods imported from China. For more information on CE vs UIAA numbers, see this article in Rock and Ice

Regarding their own labelling, the UIAA have issued this advice:

Only purchase equipment bearing the UIAA Safety Label and/or CE label. This ensures it conforms to the most exacting of international standards. The UIAA Safety, or CE, label can usually be found engraved or stitched on a piece of gear or in the product description.
Avoid buying gear online except equipment from known and trusted brands and from a trusted manufacturer or reputable retailer.

The UIAA is also developing a new Black List database where any equipment illegally advertised with the UIAA Safety Label will be recorded. For a list of all the companies whose climbing and mountaineering equipment bears the UIAA Safety Label, see here.

....Gleba pod drogą jest płaska, porośnięta trawą i bez kamieni.