One of the less important game mechanics that is fully understandable without the use of complex equations and such is Happiness. It isn't really the most helpful game mechanic in battle unless you're speaking of the moves Return/Frustration, but it is a game mechanic anyhow. The other use for happiness would be for evolution purposes, which can obviously be an important factor for stats. Anyway, lets start off with some basics.

Logic would suggest that when you catch a Pokémon, it's not going to like you, but it's not going to hate you either. There's a way to increase the Happiness of a Pokémon that you've caught immediately as well, and that's by using a Luxury Ball instead of a regular Pokeball (or any other ball for that matter). Of course, you cant use Luxury Balls on hatched Pokémon, so that shortcut is kinda useless if you are a breeder. There are lots of other ways to influence happiness however, so it's not that important.

Happiness is actually as easy to influence as eating a piece of toast or turning a doorknob, or whatever other comparisons you can think of. The things that will raise happiness include:
-Having the Pokémon in your party
-Giving it Vitamins
-Giving it a massage in Pallet Town, Goldenrod City, Veilstone City, Castelia City, Cyllage City or Ribbon Syndicate
-Giving it Certain Berries
-Gaining a Level
-Join Avenue Items
-Various juices in the Juice Shoppe

And of course, there are things that will frustrate a Pokémon, causing it to dislike you. These actions include:
-Feeding it bitter food
-Letting it faint continuously in battle

If you capture your Pokémon in a Luxury Ball, the increase in happiness is much larger than if in a normal ball.

There's also one item that influences happiness; the Soothe Bell. As for what it does, it increases the amount of happiness gained.

So you're probably wondering (or already know) how to check on your Pokémon's happiness without a fancy equation or whatever. Well, you can easily check on it at one of the houses in Verdanturf Town (Ruby/Sapphire/Emerald), Daisy in Pallet Town (FireRed/LeafGreen) or a girl in Goldenrod City. There's a lady in one of the houses in that town that will enlighten you on how much your Pokémon likes you. In Unova, there are two people who will give you a Happiness check. In Nacrene City is one who will give you a broad spectrum while in Icirrus City's Pokémon Fan Club is a woman who will give you the details. In Black 2 & White 2, Bianca will give you details too via the Xtransceiver. In X & Y, it's given by a girl in Laverre City The statements are as followed

Statement Values
X & Y: It's amazingly friendly toward you! It must be so happy spending every day with you!
B2W2 Bianca: What great friends you are! It's so nice how you trust each other! It makes me kinda jealous!
BW/B2W2: It is super friendly to you! I'm a bit jealous!
DPHGSS: It looks really happy! It must love you a lot
Plat: It simply adores you! Why, I feel like I'm intruding!
FRLG: It couldn't possibly love you any more than it does now. Your Pokemon is happy beyond words.
RSE: It adores you. It can't possibly love you any more. I even feel happy seeing it.
X & Y: You must really like your and always keep it by your side!
B2W2 Bianca: You two sure get along great! It looks like you are having fun being together! You seem bright and cheerful!
BW/B2W2: It is quite friendly to you! You must be a kind person!
DPHGSS: I get the feeling that it really trusts you
Plat: It's very friendly toward you. I can tell you treat it kindly.
FRLG: It looks very happy.
RSE: It seems to be very happy. It obviously likes you a whole lot.
X & Y: Well, I think you and ____ will be an even greater combo someday!
B2W2 Bianca: You're starting to get to be friends. Just maybe, walking along the same path has made you understand one another.
BW/B2W2: It is friendly to you. It must be happy with you.
DPHGSS: It's friendly toward you. It looks sort of happy
Plat: It's quite friendly to you. It must be happy being with you.
FRLG: It's quite friendly with you. Keep being good to it!
RSE: It likes you quite a lot. It seems to want to be babied a little.
X & Y: It's a little bit Friendly to you... Something like that.
B2W2 Bianca: You still have room for improvement. How nice! I mean, you can become even better friends!.
BW/B2W2: It is a little friendly to you... That's what I'm getting.
DPHGSS: It's quite cute
Plat: It's warming up to you. That's my impression.
FRLG: It's warming up to you. Trust must be growing between you.
RSE: It's getting used to you. It seems to believe in you.
X & Y: Hmm...I think you have a lot of time ahead of you to get to know one another better
B2W2 Bianca: You aren't getting along, are you? It's glaring at you with a look that's kinda scary
BW/B2W2: The relationship is neither good nor bad... It looks neutral.
DPHGSS: You should treat it better. It's not used to you.
Plat: It's feeling neutral toward you. It's up to you to change that.
FRLG: It's not quite familiar with you yet.
RSE: It's not very used to you yet. It neither loves nor hates you.
X & Y: Don't tell me! Are you just letting it get knocked out in Pokémon battles?!B2W2 Bianca: You aren't getting along, are you? It doesn't look like it's having fun even when you're together
BW/B2W2: Hmmm... It may not like you very much.
By any chance, you... Are you a very strict person? I feel that it really doesn't like you...
DPHGSS: It doesn't seem to like you at all. It looks mean
Plat: Hmm... It seems to dislike you a little.
FRLG: I don't like the way it glares at you. Could you try being a little nicer to it?
RSE: It's very wary. It has scary viciousness in its eyes. It doesn't like you much at all.

In Diamond, Pearl & Platinum, there is a Pokétch application that will show you the happiness of your Pokémon in your party simply by tapping them. It will be shown as hearts. There are various levels including one small heart, two small hearts, two big hearts and two large hearts. The two large hearts being the optimum happiness level. There is also a person, Dr. Footstep in Route 213, who'll give you the footstep ribbon if you show him a Pokémon with maximum happiness

So as for Return/Frustration, they work best when they're happiest/unhappiest respectively. Now seeing as it's so simple to gain happiness, I don't see why you'd want to use Frustration at all. Anyway, they're both Normal attacks and when the happiness is maxed out, the base attack is 104.

Finally, you have the ability to manipulate the Pokémon's happiness in Pokémon Black 2 & White 2 using the special items purchasable in Join Avenue. These items can be purchased once a day and can boost the happiness of the Pokémon drastically.

Name Store Effect
Brushing Beauty Salon The Pokemon becomes closer to you by grooming its hair or skin. - Increases by 10 points
Shampoo Beauty Salon The Pokemon becomes much closer to you by washing its hair or skin. - Increases by 20 points
Smile Cut Beauty Salon The Pokemon becomes a lot closer to you by taking care of its hair or skin. - Increases by 30 points
Beauty Cut Beauty Salon The Pokemon becomes extremely close to you by making its hair or skin beautiful. - Increases by 40 points
Royal Cut Beauty Salon The Pokemon becomes extremely close to you by making its hair or skin top-class. - Increases by 50 points
Friendly Drink Café A very delicious drink. Any Pokemon who drinks it will become a little bit happier. - Increases by 5 points
Friendly Lunch Café A delicious meal that came with a drink. Bargain! The Pokémon's happiness increases. - Increases by 10 points
Friendly Combo Café A hearty meal that comes with a side dish. The Pokémon's happiness increases greatly. - Increases by 20 points

To evolve Pokémon via happiness, you must reach 220 it by doing the above efforts, once that is done, you can evolve your Pokémon by making it raise a level either through battle or a Rare Candy. Below are all the Pokémon that evolve through Happiness and what they evolve into:

Lower Stage Pokémon Name Pokémon Name Final Stage
#042 Golbat #169 Crobat
#113 Chansey #242 Blissey
#133 Eevee #196 Espeon
Only in Daytime
#133 Eevee #197 Umbreon
Only in Night
#172 Pichu #025 Pikachu
#173 Cleffa #035 Clefairy
#174 Igglybuff #039 Jigglypuff
#175 Togepi #176 Togetic
#298 Azurill #183 Marill
#406 Budew #315 Roselia
Only in Daytime
#427 Buneary #428 Lopunny
#433 Chingling #358 Chimecho
Only in Night
#446 Munchlax #143 Snorlax
#447 Riolu #448 Lucario
Only in Daytime
#527 Woobat #528 Swoobat
#541 Swadloon #542 Leavanny

And thats all there is to this relatively simple game mechanic.

Thanks to Dragonair for writing this for us

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