//I'm new to linux, what do ?
If you want to know what it is exactly : https://en.wikipedia.org/wiki/Linux
if you want to try it, two solutions : install the distribution of your choice in a virtual machine, or try it with a live CD.

//what is a distribution ?
see : https://en.wikipedia.org/wiki/Linux_distribution

//Why is there so much distributions ?
Each distro have a purpose. Ubuntu target new users that want something that works out of the box. Other distributions like debian wants to be stable, or other distributions like arch or gentoo wants to be minimal and provide the user control on his machine.

//which distribution should I pick ?
Depends of you and what you want.
Friendly distributions for new users : Debian, OpenSuse, *buntu, Fedora...
Distributions for advanced users : Archlinux, Gentoo, LFS...

//dual boot or not ?
If you need a software that is not available in linux, it's better to dual-boot.
If you don't know if you'll like the distribution you choose, dualboot or test it in a VM before.

//but I heard wine can run windows software ?
Yes and no. It can't run every software that exist. The best is to try by yourself it it works.

//I heard about playonlinux, is there a difference with wine ?
PlayOnLinux use wine and scripts, and usually works better than wine alone if you want to install a software.