Onyxia is a pre burning crusade lvl 60 raid located in dust wallow marsh. To access onyxia it requires probably the longest attunement ever. It was designed for 40 lvl 60s but can be done with reasonable effort by 4-5 decently geared 70s. To actually beat onyxia still requires some knowledge of the instance. Onyxia drops about 180 gold, 3-4 epics and a few boe blues. So divided by 4 people thats 45g 1 nexus crystal and 1 large brilliant shard per person not to bad.
Oh heres a guide for onyxia if you need help killing her Onyxia (tactics - WoWWiki - Your guide to the World of Warcraft)
Here a guide for the attunement Onyxia's Lair attunement - WoWWiki - Your guide to the World of Warcraft