Q:

What format does Stack Overflow use for posting?

In my initial post I put about 15 spaces between col1 and col2. Why isn't it showing in my preview or post for that matter? What format does Stack Overflow use? Markdown/HTML? 
col1            col2   <---I have 15 spaces between these but they are not showing.

A:

If you surround code with backticks i.e. ` then whitespace won't be compressed. Backticks indicate that the contents are code and need to be displayed "as-is"
I've edited your original question for you. If you click on edit again you can see what I've done.
If you want to put multiple spaces between something that isn't code you can put it inside the HTML <pre></pre> tags e.g. col1     col2 or use &nbsp; instead of a space e.g. col1      col2 depending on how you want it to appear i.e. on its own or inline.

