What is better to use? "div" or "tables"

Hi all,

Im new here and I would like to know some doubts that I have. I observer that some websites are using "div" and other "tables".

What is the difference between those 2? What do I get or what results, if I use "div" or "tables" in my website.

To be honest, before I was using "tables" in my header and footer templates, and my site had a score of 95% SEO in whois.domaintools.com

But after I switched all things to "div", now my score had lowered to 85%, is that posible and could had been because of the changing code?

Some friends told me that using "div" is better for a fastest speed of the website...

I would like to hear some professional opinions.
Thanks alot to all,

All the best,