Great question Patrikios. Here's the answer:
1 - Anytime someone starts a new thread, they should see this box right underneath the title box. If you type any existing keyword, like "Ethics," you should see a popup allowing you to select that keyword. (Meaning, as you type E -T -H-I ------- at some point the popup will appear.
2 - At any point in any post, you can precede a word with a pound sign / hastag ( # ) and a tag will be created. As soon as you complete the post, you'll see that that word is now blue, and when you click on it you'll be taken to a page showing all other occurrences of that tag.
Here's where a problem can arise, however: It's also possible to create a new Tag just by typing in a random word. For example you can type into the Tags box, or just use a # sign, in front of "supercalifragilousexpialedocious" and a tag will be created. Or if you type "PD 10" instead of "PD10" then the tag PD 10 will be created, even though we're generally using PD10 (without the space) as the standard designation.
The system at this point doesn't allow us to limit tags to a particular set of choices, so over the years we've had a huge number of very specific tags which served no use.
That's something that the admins will fix periodically by pruning tags, so the users don't need to worry -- just when possible try to look at the list of existing tags page, and use existing tags where possible. Here's that complete list of all tags.