okay, wel this is probably a blonde question... but anyway

I was reading fruits basket and they call yuki

sohma-kun

and with other characters they have -kun

Does anyone know WHY the do that?

Thank you! (:

Thats easy, This is becasue In japan, its polite to call people by there last name followed by somehitng like -kun or -chan. I dont read fruits basket but my guess is Yuki is the first name and Sohma is the last name.

So they would say Sohma-kun (kun which is used for guys who are respected or close to the one saying there name.) insted of saying the first name Yuki.

Hope that helped.

It's as UnrealPie said.
In Japan, there are things known as honorifics. Unlike as with English, in other languages, sometimes it's inpolite to call people just by their first names only.

For instance, you know how in everyday English, sometimes someone would go, "Hello, Ms. Kennery!" It's like that, except in Japan, they would go, "Konichiwa, Kennery-san!"
There are many variations of the ending honorifics, including "-kun" "-chan" "-san" and "-sama", to name just a basic few. Every different honorific has a different meaning and level of respect behind it. 

In any case, if you still don't get it, need more information, or just feel like reading more into it, here's the Wiki Page:
http://en.wikipedia.org/wiki/Japanese_honorifics

Thank you!

I get it now! (:

You forgot -dono!