Basic Information
The seelie or "happy" court of the fair folk is that portion of the fairy nation that is not actively hostile to humanity. If there is such a thing as "good fairies" this is probably where they reside.
Seelie fae are often human like and usually beautiful - or at least not ugly. If they resemble animals or insects they will be clean, pretty, colourful ones that only bite if provoked. They live in beautiful places and appreciate the performing arts, good food and drink and a good party and whilst their humour may be embarrasing for the target, it is rarely harmful.
That is not to say that they are safe to be around - like all fairies they are easily offended, often for no apparent reason - and somewhat cavilier in their attitudes to the needs and wants of mortals. People who get on the wrong side of them sometimes end up transformed into animals or inanimate objects and even their friends can vanish for years on end, party themselves into an early grave or end up with wierd addictions and pyschological problems from the side effects of fairy food and drink.
When - and it is usually when rather than if - they get bored with a given mortal and break contact, the mortal can be left with cravings that last a lifetime, trapped in a world that now seems colourless and dull as against the half-remembered pleasures of fairyland.
The seelie court also feud constantly, both amongst themselves and with the unseelie court - the rules of these feuds are often arcane and milennia old, and mortals are best to avoid them if they can lest they end up not as players, or even pieces, but as something closer to cover or ammuntion.
Tradition, albiet possibly no older than Shakespeare, names the King and Queen of the Seelie Court as Oberon and Titania.
Sources
"The Other Wiki" on Seelie and Unseelie fae.