Quercetin is a bioflavonoid found in fruits and vegetables, but highest levels are found in apples and onions.
Like many other bioflavonoids, Quercetin has anti-oxidant, anti-artherogenic, and anti-carcinogenic properties. Quercetin is also neuroactive, with some of the same abilities as caffeine but less potent.
There is a divide between the effects seen in quercetin in in vitro (cell cultured) studies and in vivo (in living) studies, with cell studies showing great results that are not that amazing in humans or animals. This is mostly due to quercetin having low oral bioavailability (low percentage of the compound is absorbed and put to use), but could also be due to in vitro studies using a form of quercetin called 'quercetin aglycone' whereas this particular form is never found in the blood, even after ingested, as it it gets changed in the liver.
Many studies also note a high range of differences between people who ingest the same amount of quercetin, suggesting a large degree of variability is possible with supplementation.
Quercetin has GRAS (Generally Recognized As Safe) status, and no side-effects have yet been noted in doses of a few grams a day in either humans or animals.