It is available with or without a Google account, but you get a bit more data if you are signed in.
The premise is fairly simple. You give it a keyword or an existing site to look at, and it will show the related search terms that people are using, and the volume, as well as the competition. This is hugely important, as it can save a lot of wasted effort creating content, SEO campaigns or Adwords campaigns for terms that no-one is searching for. The competition value gives an indication of how hard it will be to get a first page ranking for that term.
The latest update is a great little improvement. If you enter an existing website address in the box, you can see how that website is performing in terms of the number of times it appears on the first page of search.
The new tool seems to be in beta at the moment as I am only seeing data intermittently.
How Do I Use this New Keyword Tool Data?
This is going to be primarily useful for sites that already have quite good visibility on Google. Use the tool to work out your approximate visibility for the searches for each of the terms you are targeting. More than 80% is going to be a good rate for first page appearances, due to differences in location and personal based targeting.You can also use your Analytics data to determine natural search or Adwords click through rates for those exact terms, but you will have to do this manually. Typical click rates for top of natural search are 2% to 4%, but there is a lot of variations, especially for brand name searches.
Now you have identified the key terms where you are getting lower visibility or click through rate, you can look at the page and work on improving both visibility and clicks. First, have a look at where you are ranking for this term. If you are showing below first place position, there is obviously an opportunity to get more traffic by increasing your search position for that search term, so use your favourite SEO techniques to boost that search engine rank.
You may decide that you want a new page to more specifically target this term, which could be a new blog post, product or service information page, which you can then promote both from your existing pages and off-site.
Lastly, look at the landing page itself. First port of call is the 'snippet' that appears beneath a search result. Make sure this is as enticing as possible for the searchers you are targeting. This is usually controlled by the Meta Keywords attribute on the page. A Good snippet will boost the click through rate no matter what the position you appear in.