Firstly, the commenter down/up there is remarkably rude, and secondly, they're objects that have had to be detained by the SCP, an "Area 51" or "Men in Black" kind of thing. It can be pretty much anything that would make a crowd scream and run in terror, or at the very least would make you say, "What the hell is this thing?"
There is no term of SCP project, SCP is an acronym for to things, Secure Contain Protect and Special Containment Procedures. Project fits no where in either of them. Than again I guess sarcasm doesn't show on the internet, my bad
To continue Retrodude's explanation: Keter Scps are difficult to contain, kill without provocation, and could cause an End-of-the-world scenario if released. Euclids are somewhat easier to contain, and have to be provoked in some way. Safe objects are objects that are inert, beneficial, or nonviolent. Containment's very easy.
You are wonderfully naive and I appreciate that on the interwebs, using the terms damage, makes me giggle. Euclid is a class used to show that they are unpredictable, sentient if you will. They are dangerous, but won't attack quite yet. Keters are different, they have oure hatred for humanity and will always attempt to kill you. Get it?
Sure! All of these symbols are submitted under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 License, which means you're free to use them for non-commercial purposes so long as you submit it under the same liscense as well as attribute me, the original artist, and the Foundation.