CSS is as a whole and it refers to the actual stylesheet language. On the other hand, CSS3 is the latest version of CSS.
Like any other software, CSS also upgraded its version three times. On December 17, 1996, W3C published the CSS that we call CSS1. See more detail about the CSS history.
Likewise, the next version is called CSS2 and then CSS3 (final/forever version). But all those are the CSS itself as a whole.
Not to mention, sometimes you may hear about CSS4 but it does not exist. One of CSSWG (CSS Working Group) members said:
They’re all CSS3 (or just CSS), regardless of what level they’re at.
So, whatever CSS code you write at this time is all CSS3 or just CSS. In this context, there is no difference between CSS and CSS3. These two are the same!
Do you need to learn all the versions
Since you have the latest version, there is no reason to go through the 1996s. Nor it has worth. However, you’ll see a few differences among versions. These are the new properties added and a few legacy components have been deprecated.
So whatever you see about versions (CSS4, CSS3, CSS2) on the internet, just ignore them and learn only CSS.
I wish, W3C will remove or drop the CSS version entirely and it should be.
This two CSS and CSS3 are the same thing.
There is no CSS4 and it does not exist. CSS4 is a rumor.
You don’t need to check for the versions, nor it’s required. Everything you’re writing in these days is CSS. Also, the CSS and CSS3 are the same. So don’t worry about its version.
If you still have any questions about CSS or CSS3 or anything regarding the versions, let me know.