When you click "create an archive", Google selects all the products that are available and shows the size of the ZIP archive you can download. Archives larger than 2GB will be split into multiple ZIP files. You can switch to .TGZ (.tar.gz) or .TBZ (.tar.bz2) tarball files, but you need a software like 7-zip to open them if you use Windows.
By default, all the products are selected. You can click "select all" and check the data you want to download. Click each product to find more about the data you download, change the format and sometimes even filter the data.
For example, you can download Drive files from certain folders and change the export formats. You can export contacts and Google+ circles as CSV, HTML or vCard and select the blogs and photo albums you want to download.
After creating an archive, you'll get an email when it's ready for download and the link will be available for a week. You can find all your archives in the "my archive" section.
"Remember, your data is important! Do not download archives on public computers. It's important that you have control over your data. If you have decided to take your data elsewhere, please research the data export policies of your destination. Otherwise, if you ever want the leave the service, you may have to leave important stuff like your photos behind," informs Google.
Update: Takeout doesn't work that well. I created 3 archives (zip, tar.gz, tar.bz2) for all the supported Google services and 2 of the archives failed at 100%. The only archive I could download included a 100 KB errors.html with a long list of files that couldn't be downloaded (most of them were Drive files Google+ posts).
"We tried our best to get your archive 100% right, but ran into a few problems as we were putting it together. Instead of canceling the entire archive, we've created this list of items that weren't properly included. Click any of the names to be taken directly to the item in question, and you can download it from there," informs the page.
I also noticed that not all my YouTube videos were downloaded. For some of them, YouTube downloaded HTML pages which mention that they were matched by the Content ID system. "Because someone else has claimed the copyright on some, or all, of your video YouTube has a policy of not allowing this video to be exported." It's probably because I used YouTube's "add music" feature.
{ Thanks, Florian K. }