The sylvari are a young race, not yet born in any of the original games. They are born fully grown from the pale tree, which was planted by the centaur Ventari
The sylvari are first hinted at during the end cinematic of Eye of the North, when the voiceover mentions "new creatures would stir" and the view pans over the grove where the pale tree is growing.
In GW2, the sylvari are still learning about the world. They're curious about everything and everyone, and seek knowledge above all else. Before they're 'born' the sylvari are taught the basics of life in their dream. When they sleep, their experiences are shared with other sylvari in this dream of dreams. This is an imperfect group mind, where each sylvari only recieves bits of the whole, so not everything one sylvari knows will be known by all others.
It's not all sunshine and roses for the sylvari, where there are dreams there is also the nightmare. The nightmare court is a dark group of sylvari that follow the nightmare instead of the dream.
I like the sylvari. A lot of people compare them to elves, but I think they're pretty much the opposite - elves are old and wise, sylvari are young and curious. They're also plants, which is pretty unique. My only concern would be that from what I have seen of them, they seem to be naive green humans with a hive mind. I'm hoping that when we finally see more of them, their design will have become as unique as their story.
Read more about the sylvari on guildwars2.com's sylvari page.