Can anyone tell me what these two sections of code are for. Why is there html, body and another one called body

html, body { margin: 0;
padding: 0;
}


body { padding-bottom: 20px;
text-align: center;
}


#container {
text-align: left;
margin: 0 auto;
width: 780px;
}


#header {
border: 1px solid black;
margin: 20px;
padding: 20px;
}


#sidebar {
border: 1px solid black;
width: 300px;
margin-left: 20px;
float: left;
display: inline;
}



#content {
margin-left: 342px;
margin-right: 20px;
border: 1px solid black;
padding: 20px;
}


#footer {
margin: 20px 20px 0 20px;
padding: 20px;
border: 1px solid black;
clear: left;
}