Laughter is an audible expression (written as ha ha ha or lol etc.), or appearance of merriment or happiness, or an inward feeling of joy and pleasure (laughing on the inside). It may ensue (as a physiological reaction) from jokes, tickling, and other stimuli. Inhaling nitrous oxide can also induce laughter; other drugs, such as cannabis, can also induce episodes of strong laughter. Strong laughter can sometimes bring an onset of tears or even moderate muscular pain.
Laughter is a part of human behaviour regulated by the brain. It helps humans clarify their intentions in social interaction and provides an emotional context to conversations. Laughter is used as a signal for being part of a group — it signals acceptance and positive interactions with others. Laughter is sometimes seemingly contagious, and the laughter of one person can itself provoke laughter from others as a positive feedback.[1] An extreme case of this is the Tanganyika laughter epidemic. This may account in part for the popularity of laugh tracks in situation comedy television shows

Laughter, makes wonders in different situations. When to use it:

T.V. and Movies: There’s no shortage of laughter opportunities from the entertainment, both at the theater and in the aisles of the video stores, as well as at home with T.V. comedies. While wasting your time watching something marginally funny may actually frustrate you, watching truly hilarious movies and shows is an easy way to get laughter into your life whenever you need it.
Laugh With Friends: Going to a movie or comedy club with friends is a great way to get more laughter in your life. The contagious effects of laughter may mean you’ll laugh more than you otherwise would have during the show, plus you’ll have jokes to reference at later times. Having friends over for a party or game night is also a great setup for laughter and other good feelings.
Find Humor In Your Life: Instead of complaining about life’s frustrations, try to laugh about them. If something is so frustrating or depressing it’s ridiculous, realize that you could ‘look back on it and laugh.’ Think of how it will sound as a story you could tell to your friends, and then see if you can laugh about it now. With this attitude, you may also find yourself being more lighthearted and silly, giving yourself and those around you more to laugh about. Approach life in a more mirthful way and you’ll find you’re less stressed about negative events, and you’ll achieve the health benefits of laughter. (See this article on maintaining a sense of humor.
‘Fake It Until You Make It’: Just as studies show the positive effects of smiling occur whether the smile is fake or real, faked laughter also provides the benefits mentioned above. So smile more, and fake laughter; you’ll still achieve positive effects, and the fake merriment may lead to real smiles and laughter.

Scientific explanation:
Modern neurophysiology states that laughter is linked with the activation of the ventromedial prefrontal cortex, which produces endorphins after a rewarding activity.

Research has shown that parts of the limbic system are involved in laughter[citation needed]. The limbic system is a primitive part of the brain that is involved in emotions and helps us with basic functions necessary for survival. Two structures in the limbic system are involved in producing laughter: the amygdala and the hippocampus. The December 7, 1984 Journal of the American Medical Association describes the neurological causes of laughter as follows:
“Although there is no known ‘laugh center’ in the brain, its neural mechanism has been the subject of much, albeit inconclusive, speculation. It is evident that its expression depends on neural paths arising in close association with the telencephalic and diencephalic centers concerned with respiration. Wilson considered the mechanism to be in the region of the mesial thalamus, hypothalamus, and subthalamus. Kelly and co-workers, in turn, postulated that the tegmentum near the periaqueductal grey contains the integrating mechanism for emotional expression. Thus, supranuclear pathways, including those from the limbic system that Papez hypothesised to mediate emotional expressions such as laughter, probably come into synaptic relation in the reticular core of the brain stem. So while purely emotional responses such as laughter are mediated by subcortical structures, especially the hypothalamus, and are stereotyped, the cerebral cortex can modulate or suppress them.”

Interesting facts, animals can laugh:

Sure, life can be funny, and not just for humans beings. Animals laugh too!
Studies by different groups suggest dogs, monkeys and even rats love a good laugh. People, meanwhile, have been laughing since before they could talk.
Acording to scientists neural circuits for laughter exist in very ancient regions of the brain, and ancestral forms of play and laughter existed in other animals eons before we humans came along with our ‘ha-ha-has’ and verbal repartee. Even when chimps play and chase each other, they pant in a manner that is strikingly like human laughter.
Dogs have a similar response.
Rats chirp while they play, again in a way that resembles our giggles. Panksepp found in a previous study that when rats are playfully tickled, they chirp and bond socially with their human tickler. And they seem to like it, seeking to be tickled more. Apparently joyful rats also preferred to hang out with other chirpers.
More study is needed to figure out whether animals are really laughing. The results could explain why humans like to joke around. And there are speculations it might even lead to the development of treatments for laughter’s dark side: depression.
Meanwhile, there’s the question of what’s so darn funny in the animal world, although no one has investigated the possibility of rat humor, if it exists, it is likely to be heavily laced with slapstick. Even if adult rodents have no well-developed cognitive sense of humor, young rats have a marvelous sense of fun.
Science has traditionally deemed animals incapable of joy and woe, although some still regard laughter as a uniquely human trait, the joke’s on them. There is ideed lougter in animal world and it might indicate a close genetic relation of animals that manifest that sort of behavior to human kind since humans, as forementioned exibit similar kind of behavior youngest stages of their life.

Article sources: (1) http://en.wikipedia.org/wiki/Laughter; (2) http://www.chilloutpoint.com/featured/laughter.html

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