In a CSS selector, I do not understand the meaning of a space between a layout element and an id (or class):

CSS   

```
aside#sidebar{
    float:right;
    width:30%;
    margin-top:10px;
    color:#ffffff;
}

```

```
aside #sidebar{
    float:right;
    width:30%;
    margin-top:10px;
    color:#ffffff;
}

```

HTML  

```
<aside id="sidebar">
    <h3>What We Do</h3>
    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</p>
</aside>

```

Thanks