There are 7 OS people normally use with Pi.
The most suitable OS for beginners. Easy to use, excellent support and also it is fast.
A full desktop OS with heaps of software preinstalled. Slow and big.
SNAPPY UBUNTU CORE
This is a cloud OS designed for Docker.
Like the name suggests, it's a CORE with very basic features. The boot time is about 1 minute.
If you want to use Wifi directly, you'd take a few steps cause there is no editor on it, so you can't edit the wifi config on your Pi directly.
Even if you connect to network via ethernet, you have to set the system time as the current real time to install any packages, e.g.
sudo date -s "Sat Feb 7 09:57:32 GMT 2015", otherwise would get pycurl.error.
There is no Gui.
This OS is for Docker app or IoT.
OPENELEC / OSMC
Both are Media center.
Based on Fidora, but the lastest release is 2014, so no ARMv7 version.
Not Linux, some old stuff.
Used to be on the raspberrypi.org downloads page, but not anymore.
You need a linux machine with root (not just sudo) to install the .tar.gz file to your (micro) SD card.
Very fast. The boot up time only takes about 10 seconds.
But the document is not so organised for Pi. You need to do lots of things by yourself.
(For the lxde, you might need the xf86-video-fbdev pacakge installed)