They are pretty similar in terms of function. The first forces a div to the full height of the page and then give it a negative margin the size of the footer.
html, body {
height: 100%; /*set 100% height*/
}
.wrapper {
min-height: 100%; /*content 100% height of page */
height: auto !important;
height: 100%;
margin: 0 auto -142px; /* negative value causes wrappers height to become (height of page) minus 142px, just enough room for our footer */
}
.footer, .push {
height: 142px; /*Footer is the footer, push pushes the page content out of the footers way*/
}
What this does is makes sure that all content within the wrapping div is 100% of the page height minus the height of the footer. So that as long as the footer is the same size as the negative margin it will stick in the gap left and appear at the bottom of the element.
The second also forces the content to be 100% of the height of the page.
html, body {height: 100%;} /*set 100% height*/
#wrap {min-height: 100%;} /* make content 100% height of screen */
It then creates a space at the bottom of the main content the same size as the footer.
#main {overflow:auto;
padding-bottom: 150px;} /* wrapper still 100% height of screen but its content is forced to end 150px before it finishes (150px above bottom of screen) */
Then using position relative and a negative top margin forces the footer to appear 150px above its normal position (in the space it just made).
#footer {position: relative;
margin-top: -150px; /* Make footer appear 150px above its normal position (in the space made by the padding in #main */
height: 150px;
clear:both;}
Note: This only works so long as your page content is kept within .wrapper and #main inside #wrap respectively, and your footer is outside of these containers.
If you didn't understand any part of that leave me a comment and I'll try to answer it.
Edit: In response to user360122
HTML markup for first:
<html>
<body>
<div class="wrapper">
<!--Page content goes here-->
<div class="push">
<!--Leave this empty, it ensures no overflow from your content into your footer-->
</div>
</div>
<div class="footer">
<!--Footer content goes here-->
</div>
<body>
</html>
HTML markup for second:
<html>
<body>
<div id="wrap">
<div id="main">
<!--Page content goes here-->
</div>
</div>
<div id="footer">
<!--Footer content goes here-->
</div>
</body>
</html>
Remember to include the stylesheet and declare doctype .etc (these aren't full html pages).