Categories vs. tags

Mr C and I had a bit of a barney last night. He’s been sorting out the categories on my website, making it possible to stick entries in more than one. So, actually, I think these are tags. But he was adamant that they’re the same thing. Category = Tag. Tag = Category. Well, actually, Dictionary.Com says:

Category = A specifically defined division in a system of classification; a class.
Tag = To label, identify, or recognise with or as if with a tag.

In my opinion, you can only put things in one category. Like if you had a stack of papers and you had to file them in a category, you would only be able to put them in one place or another. But if you had a stack of papers to tag, then it would be sticking labels on them and you could that as many times as you like.

I guess essentially it’s the same thing, it’s just a filing system and it doesn’t really matter what you call it, but I just want to get my head around the fact that the two words mean different things. I’m not that good at explaining things, but that’s how I see it. I still don’t think he believes me.