CSS is stand for cascading style sheet. 
CSS is used to Structure the style of a web document in a simple and easy way. 
CSS is used to define styles for your web pages. 
CSS Syntax
The selector shows to the HTML element you want to style. 
The declaration block consists of at least one declaration isolated by semicolons. 
Each declaration should contain a CSS property and a value, separated by a colon. 
Example 
<html>
    <head>
    <style>
        p{
         color: red;
         text-align: center;
         font-family:tahoma;
        font-size: 30px;
   
        }  
    </style>
    </head>
<body >
    <p>Hello World </p>
   
</body>
</html>
Output 

    
    
    

    
Hello World