I think Yosemite is the ugliest Mac OS that Apple has ever released (and I have been there since the beginning). I think the look-and-feel is appalling, and I cannot stomach it ... so have reverted to Mavericks.
If you do not have a Time Machine backup, then there are two main options:
1. Command-R: Recovery system
If you have a newer Mac, when you boot up your Mac, hold down Command-R ... this allows you to install a new copy of OS X Mavericks which it will download from the Apple servers on the internet.
Downside: it does not let you install Mavericks when you have Yosemite already installed, so you need to Erase your HD using Disk utility (make a backup first, of course), and then re-boot your Mac, hold down Command-R, and install a fresh system - you end up with a brand new Mac.
For more detail, see: http://support.apple.com/en-us/HT4718
2. Bootable Mavericks USB stick
In theory, if you have Mavericks installed as a bootable USB stick, you should be able to install Mavericks from that. It didn't actually work for me, perhaps because my version of Mavericks on the USB stick (Jan 2014) came out before the date I purchased my Macbook Air ... and I had to use the recovery system which worked fine ... albeit starting clean from fresh.
For help creating a bootable Mavericks install USB stick:
There is also an article in Macworld on how to revert to Mavericks from Yosemite: