What do the suffixes -kun, -chan, -san and -sama mean?

They are Japanese honorifics. They are used like suffix (they are added to the end of a name) to show respect.

Kun is mostly used toward young men/boys.

Chan is considered to be very feminine. It is used among close female friends or family, for the most part.

San is the most common and it's used with guys and gals.

Sama is the formal version of san. It is used when speaking to someone older than you or someone in a higher rank.

Well, "-kun" normally stands for " Mr ". They refer to boys in that way, like " Li-kun ", so it's for the men.
" -chan " it's a suffix you use for someone close to you, like your best friend, or your pet, etc. But you use it only when you call them by their first name. Note: Boys' will be the same, "-kun".
"-san" is for someone you think is higher than you, like the way Sakura calls Yukito " Yukito-san". Mainly, it's to show you have respect for that person.
"-sama" is for someone that is of royalty, or you respect till the ends of the earth.
There's still many, like "-senpai" , "-ou". But I'm not too sure. But when you call a person without the suffix, that's means you're VERY close to him/her, like the way Shaoran calls Sakura with out the suffix when he uses her first name.

Okay! I think that's it. Forgive me if I'm wrong anywhere, I'm still a kid and not too bright. lol. Matte ne!

Actually san is mostly used for addressing a stranger and sama is right but also used for master

-kun is used for boys. Example: Nina called Mitsuko's name by "Mitsuko-kun!" when she was excited. -chan is mostly used for girls. But for little boys too! Also for pets. It mostly stands for childish cuteness. Example: Mitsuko said "Nina-chan", when he showed her Amy. -san is like in a polite situation. Example: Mitsuko said Nina-san when he greeted her by saying, "Hello, Nina-san." -sama means in a much more polite situation. Example: Mitsuko said "Nina-sama" when he needed her help. Ask me for more, later on.

-kun is used for someone close to u and it's used for boys (i think)

-chan is used for girls and cute stuff

-san is more formal

-sama is used for someone who is higher ranks than u

-senpai is for someone a little older or a grade/year higher than u