To most, there are just 3 Classes know, Safe, Euclid, Keter. But there are 5 offical classes. Safe, Euclid, Keter, Thaumiel, and Apollyon. Also known as, SEKTA.
Class Safe : Safe-class SCPs are anomalies that are easily and safely contained. This is often due to the fact that the Foundation has researched the SCP well enough that containment does not require significant resources or that the anomalies require a specific and conscious activation or trigger. Classifying an SCP as Safe, however, does not mean that handling or activating it does not pose a threat.
Class Euclid : Euclid-class SCPs are anomalies that require more resources to contain completely or where containment isn't always reliable. Usually this is because the SCP is insufficiently understood or inherently unpredictable.
Euclid is the Object Class with the greatest scope, and it's usually a safe bet that an SCP will be this class if it doesn't easily fall into any of the other standard Object Classes.
As a note, any SCP that's autonomous, sentient and/or sapient is generally classified as Euclid, due to the inherent unpredictability of an object that can act or think on its own.
Class Keter : Keter-class SCPs are anomalies that are exceedingly difficult to contain consistently or reliably, with containment procedures often being extensive and complex. The Foundation often can't contain these SCPs well due to not having a solid understanding of the anomaly, or lacking the technology to properly contain or counter it. A Keter SCP does not mean the SCP is dangerous, just that it is simply very difficult or costly to contain.
The rest of the SCP Classes are not seen(yet) in our site(s)
Class Thaumiel : Thaumiel-class SCPs are anomalies that the Foundation uses to contain or counteract other SCPs or anomalous phenomena. Even the mere existence of Thaumiel-class objects is classified at the highest levels of the Foundation and their locations, functions, and current status are known to few Foundation personnel outside of the O5 Council.
Class Apollyon : A translation of the name, Abaddon, in Herbrew 'The Angel of Death'. An SCP that is nearly impossible to contain/recontain, it's properties pose a serious and immediate threat to the human life.
Class Neutralized : Neutralized SCPs are anomalies that are no longer anomalous, either through having been intentionally or accidentally destroyed, or disabled.
There are also other SCP Classes, but that are not confirmed/offical classes. But it does not change the fact some Foundation sites use them.
Class Explained : Explained SCPs are commonly articles about anomalies that are completely and fully understood to the point where their effects are now explainable by mainstream science or phenomena that have been debunked or falsely mistaken as an anomaly.
Class Maksur : A SCP that is broken, or damaged, for certain reasons, The Broken God is a example of this Class, SCP.
Class 53 : A SCP that is nothing known about, at all, but, has proven to be not dangerous yet either. This class is mostly refered to Scantron's Proposal SCP-001
Class Truculent : (This class has offically not been published, but i did still want to put it in) This is a Class for SCPs that are very dangerous to interact with.
Class Simpatico : (Also not published) A Class for SCPs that, even though rather unpredictable, it is rarely a serious threat to living beings (Humans)