We won't know what he's offering for players until he appears, but we do know that he'll offer a variety of items. He used to appear on Io and Titan, but both of those locations are now in the 'Destiny Content Vault'. We say random location, but he'll always be in the same spots you've always found him in - 'Winding Cove' in the EDZ, in a tree in 'Watcher's Grave' on Nessus, or in the Hangar in the Tower. He appears at 5 pm GMT (6 PM BST) in a seemingly random location and offers rare gear in exchange for legendary shards that you can collect from dismantling legendary gear or completing activities.
If you're new to the game, Xur is a character that sells exotic weapons and armour over the weekend.