As koolprasad2003 says:
"Couple of DIV's with images only. nothing more than that. Where is the definition for CSS classes used in DIV"
More explanation is not going to be forthcoming: there isn't anything to explain other than "raw" HTML - and that would be a line-by-line explanation.
Do you have any idea how much work explaining code line by line is?
Every single line needs a paragraph of explanation! For example:
int next = r.Next();
Create a new variable called "next" which can hold a integer value. From the previously declared Random instance "r", call the "Next" method to get a new random number, and assign it to the "next" variable.
Can you imagine how long it would take us to explain even a very short code fragment like your example, line by line?
No. It is not going to happen. If you have a specific problem, then ask a question about it. But think first - would you want to sit down for 45 minutes and type up a line-by-line description for no good reason?
And almost certainly, if you look carefully at the rest of the page, it's the accompanying javascript that does the work. And before you ask: no, we won't explain that to you line-by-line either!