Understanding of pseudo classes

Pseudo classes have been learned for some time , But I have been confused , Today I have some understanding .

CSS Pseudo class is a special effect used to add some selectors .
Use it in the tag name / After class name + The colon + Pseudo class name + Pseudo class
such as a:hover {color:#FF00FF;} /* Mouse over link */
Commonly used ：

a:link {
    color:#FF0000;} /*  Links not visited  */
a:visited {
    color:#00FF00;} /*  Links visited  */
a:hover {
    color:#FF00FF;} /*  Mouse over link  */
a:active {
    color:#0000FF;} /*  Selected links  */

p:before
{
    
	content:"aaaaa";
}
/* Add... Before the paragraph  aaaaa*/

. Class name ：after{
    
content: "bbbbbb"; 
}
/* Generate bbbbbb As the last element */