testing wordpress site

Hi.

How can I test / tweak the whole wordpress site, without it being visable to the public?

I know that you can do this with individual pages etc, but what about the whole site?

If this is not an option – is there a way to test and tweak in a different folder say…www.mysite.com/test and then move to http://www.mysite/

I hope you can see my point – the site would be live as it is being constructed – is there anyway around this?

Thanks for your time in advance.