Differences between CSS () and addclass () in jquery learning beginners

Getting started with jquery: CSS () and addclass ()
I have been studying jquery in the past two days. I just started to get started. from yesterday, I found a method of addclass. In this tutorial, a method is written like this.
$ (Document). Ready (function (){
$ ("P"). addclass ("red ");
});
However, I did not take this into consideration and thought that red is the specific color of red. Then, when I write an example myself, I define a $ ("p"). addclass ("green ");
After the operation, I did not find any changes in CSS. Today I carefully studied my God. In this example, red is a type that has been defined in CSS, But I mistakenly think it is red color, the reason is that you do not know much about CSS. Then I defined a. Green {color: green;} style in the CSS file. I tried it and it was OK.
For CSS (), this method is defined to set the value of a style attribute among all matching elements,
Example: $ ("P" ).css ("color", "Red"); Prefix: attribute name, followed by specific attribute parameters.
I decided to send all my learned knowledge to my blog, which gave me a little sense of accomplishment ......
Now, continue with my jquery