COPY PASTED FROM DA, written for anime beginners, I suppose.

A small reference and mini guide from yours truly ♥
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Ah, eyes. Possibly one of, if not THE most important feature in any anime character. I'd say it's the most important. Eyes in anime usually tell a lot about a character. They can determine exactly what type of person your character is. To me, they're probably the most fun part to draw. There are so many ways to do it, and believe it or not, the slightest change in pupil and shine size, shading, and shine direction can make a huge difference in your eyes.
I'll go through different eye styles here and list some suggestions on how to achieve them, according to personality.

GENTLE- There are different ways to do this. The way I prefer to do this is to draw the eye slanted downward, and avoid small, defined pupils. The eye is either filled in completely or left with the popular crescent moon iris, and add some small shine here and there. This will give your character an innocent, dreamy look, which is good for shy, gentle, healing-type characters, such as for example, a quiet and reserved, sweet school girl or something. This works best for female characters however if you're a fan of cute guys, this can also work. This is how I draw Donatello's eyes, anyway, since they match his shy, caring, giving personality in my opinion.

BADASS-When you want a more masculine, perhaps badass character, reducing the eye size is the first step. Bigger eyes will always make your character look cuter, regardless of how its drawn. Defining the pupil (making a black circle in the middle) and adding little or no shine can add a less adorable effect, and sometimes even scary O___O;! Making the eye shape less round and more sharp at the ends adds mystery, and perhaps elegance, depending on how it's drawn. I find this style to work best for guys, however it can also work for sexier females. This is pretty much how I draw Raph's eyes cuz he's the hotheaded bad boy, so slanted eyes with defined pupils brings that rebellious personality out.

ELEGANT-I've found that using an almond eye shape gives a sort of elegance to a character. Not sharp, yet not round. It's hard to describe, but a good example of this eye shape is Azusa from the anime K-On! She's a mature girl, and her character shows through her almond-shaped eyes. This eye style is well suited for that kind of character-mature, mysterious, elegant, perhaps quiet. Either defining the pupil or not gives the same sense of maturity in the character. This is how I draw Leo's eyes, so his are a good example. Him being the older brother and leader, I thought this was the correct style for an anime rendition of him ^^

CUTE-Cute is achieved easily! You see the style all the time...the most common anime eye style is the basic, round eye. Defining the pupil adds life and energy to your character, while simply using the crescent iris gives your character a more adorable look. And of course, adding shine, and making the eyes bigger only means one thing-MORE CUTE! This is how I draw Mikey's eyes, with the pupils defined, because it fits with his lively,energetic, and happy attitude.

And some other things you already KNOW...
-Bigger shine makes your character look more cartoony. Overdoing shine is a no-no in serious pictures, this works best when it's meant to be cute and cartoony.
-Making the pupil and iris smaller also makes your character look more cartoony. A good example of this is the Pokemon anime series, One Piece, and the Sonic characters :3.
HOWEVER, depending on how you draw the overall eye shape, it can also give your character an almost smug, and badass effect that works very well for guy characters.
-In many anime, leaving out the shine and iris completely and filling in the eye one color means the character is hypnotized, or brainwashed, or dying. Basically, the life is kind of sucked out of your character.
-Usually, adding shine at the very top can also add a more cartoony effect, whereas adding it to the side adds a more "pretty" effect.
-Making the top eyeline thicker makes your character look more doll-ish, but it works best when the main eye itself is round and not completely connected to the bottom eye line, however this is just my opinion, experiment!

My brain just exploded, so, I'll just cut it short. I might make an in-depth tutorial in the future if anyone is interested, I just wanted to give a brief idea of how eyes are used in anime :P