He was all sorts of things; explorer, commander, adventurer, trader - he circumnavigated the globe in the late 1570s, and commanded a portion of the English fleet during the action with the Spanish Armada, hence the knighthood. First and foremost however, he was a privateer, which basically means that he was indeed a pirate. The crucial difference, however, is that was authorized by the crown to conduct his piracy, which was aimed at the Spanish, for which they called him all sorts of names. He was operating during the first Anglo-Spanish War, which incidentally was also at the height of Spanish dominance of the New World. Occasionally he would go rogue and commit piracy against non-Spanish vessels (i.e. Portuguese, French etc.) but, most of the time, his prizes were fair game. I can understand why the Spanish may have been slightly sore about it at the time, because from their perspective privateering is just the same as independent piracy when its authorized by your arch-nemesis.

Hope that's the sort of answer you were after.