Relative positioning and Absolute positioning

Relative positioning

Relative positioning is a fairly easy concept to grasp. If you relatively position an element, it will stay exactly where it is. You can then shift the element relative to its starting point by setting a vertical or horizontal position. If you set the top position to be 20 pixels, the box will appear 20 pixels below the top of its original position. Setting the left position to 20 pixels will create a 20- pixel space on the left of the element, moving the element to the right (see Figure).

#myBox {
position: relative; left: 20px;
top: 20px;
}

With relative positioning, the element continues to occupy the original space, whether or not it is offset. As such, offsetting the element can cause it to overlap other boxes.

See: How to align 3 divs side by side horizontally (left center right) inside another div?

Absolute positioning

Relative positioning is actually considered part of the normal flow-positioning model, as the position of the element is relative to its position in the normal flow. By contrast, absolute positioning takes the element out of the flow of the document, thus taking up no space. Other elements in the normal flow of the document will act as though the absolutely positioned element was never there (see Figure).

An absolutely positioned element is positioned in relation to its nearest positioned ancestor. If the element has no positioned ancestors, it will be positioned in relation to the initial containing block. Depending on the user agent, this will either be the canvas or the HTML element.
As with relatively positioned boxes, an absolutely positioned box can be offset from the top, bottom, left, or right of its containing block. This gives you a great deal of flexibility. You can literally position an element anywhere on the page.

The main problem people have with positioning is remembering which type of positioning is which. Relative positioning is “relative” to the element’s initial position in the flow of the document, whereas absolute positioning is “relative” to nearest positioned ancestor or, if one doesn’t exist, the initial container block.

Because absolutely positioned boxes are taken out of the flow of the document, they can overlap other elements on the page. You can control the stacking order of these boxes by setting a property called z-index. The higher the z-index, the higher up the box appears in the stack.
Positioning an absolutely positioned element in relation to its nearest positioned ancestor allows you to do some very interesting things. For instance, say you wanted to align a paragraph of text at the bottom right of a large box. You could simply give the container box a relative position and then absolutely position the paragraph in relation to this box:

#branding {
width: 70em; 
height: 10em; 
position: relative;
}
#branding .tel { 
position: absolute; 
right: 1em;
bottom: 1em; 
text-align: right;
}
<div id="branding">
<p class="tel">Tel: 0845 838 6163</p>
</div>

Absolute positioning can be a useful tool when laying out a page, especially if it is done using relatively positioned ancestors. It is entirely possible to create a design solely using absolute positioning. For this to work, these elements need to have fixed dimensions, so you can position them where you want without the risk of overlapping.
Because absolutely positioned elements are taken out of the flow of the document, they have no effect on boxes in the normal flow. If you were to enlarge an absolutely positioned box—by increasing the font size, for instance—the surrounding boxes wouldn’t reflow. As such, any change in size can ruin your finely tuned layout by causing the absolutely positioned boxes to overlap.

By CSS Mastery…