Difference between <p> and <span>

Difference between <p> and <span>

HTML

<div class="colors">
    <h3>Display inline</h3>
    <span class="red">4</span>
    <span class="blue">5</span>  
    <span class="green">6</span> 
</div>

CSS

h3 {color:#000;}
body {font-family: 'Arial', sans-serif;}
.colors {
    text-align:center;
    color:#fff;
}
.red {
    background-color:red;
}
.blue {
    background-color:blue;
}
.green {
    background-color:green;
}
span {padding:5px;}