Authors have long been using tables, floats, inline-blocks, and other CSS properties to lay out their site content. However, none of these tools were designed for the complex webpages and webapps we are making nowadays. Simple things like vertical centering require work. Complex things like flexible grid layouts are so hard that it’s considered ambitious to roll your own, hence the success of CSS grid frameworks. Still, if so many projects needs to do these things, why can’t it just be easy? Flexbox aims to change all that.
Its going to be great to see this take over the internet in the not to distant future. If you want a head start then this article by Greg will help you lead the pack.