There is very little official fluff regarding Exodites, most of wha tyou'll find out there is stuff people have made up. They are Eldar that sparated from the rest before the fall, seeing the direction their society was headed and getting out of it. They live on worlds that the Eldar terraformed or preserved. Exodites live a very simple lifestyle, having seen the result of technological decadence. Often the word "primitive" is assigned to them, although thast mebbe a bit of an exaggeration. THey have a clan-like social grouping, led by something akin to a shaman (I don't remember what they are called offhand, its a sort of underdeveloped seer). They ride "dragons" (think WFB 5th Ed. Cold Ones) into battle as cavalry, fly around on Terradons, etc. CWE and DE usually ignore them as useless and simple, Exodites think the rest of us are too uppity and enslaved to technology.
Thats the basics, as Rasmus mentioned look through this forum for more info. They aren't as popular as the Quins, and no official list has ever even been attempted so far as I know, they are more of a fluff gimick than anything. A number of people have made their own lists, they aren't too hard to find using Google or sumthin.
Rils
PS - You might want to consider narrowing yer question a bit, "tell me everything" is a bit ambitious especially concerning Harlequins. Taking a few minutes to do some research will answer that question a lot better than a directionless request. Then if there are specific areas you are interested in, post specific questions that people can answer. Good luck with your search! ~R~