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 mostrar respect.

Kun is mostly used toward young men/boys.

Chan is considered to be very feminine. It is used among close female friends o 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 tu o 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 tu use for someone close to you, like your best friend, o your pet, etc. But tu use it only when tu call them por their first name. Note: Boys' will be the same, "-kun".
"-san" is for someone tu think is higher than you, like the way Sakura calls Yukito " Yukito-san". Mainly, it's to mostrar tu have respect for that person.
"-sama" is for someone that is of royalty, o tu respect till the ends of the earth.
There's still many, like "-senpai" , "-ou". But I'm not too sure. But when tu 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 por "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 dicho "Nina-chan", when he showed her Amy. -san is like in a polite situation. Example: Mitsuko dicho Nina-san when he greeted her por saying, "Hello, Nina-san." -sama means in a much más polite situation. Example: Mitsuko dicho "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 más formal

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

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