Class in CSS

       * you can give your class any name you want . I will be calling 'classname' here.

then you can define it in your css file like below .

div .classname{
 text-align:center;
 background-color:#ffc;
 font-weight:bold;
}

2, >>   I want to also know difference between class and div in CSS ?

I don't know what's your mean.  I guess you want to understand the difference between 

div .classname and .classname  and .classname div ?

div .classname : refers to all divs with the class 'classname' 
.classname div : refers to all divs that are children of the element with the class 'classname' 
.classname      :    refers to all elements with the class 'classname'