Cards Discarded from your hand and also considered 'Sent', however, 'Sent' cards are not automatically classed as Discarded. Similarly, when a card is destroyed it is considered 'Sent', however, 'Sent' cards are not automatically considered 'destroyed'ref.
Proper knowledge of the difference between 'Sent' and 'Destroyed' could mean the difference between a win and a loss in playing the game. For example, some of the Dark World series of monsters have their effects activate when they are discarded to the graveyard. They activate with the resolution of Card Destruction but not Hand Destruction because the former 'discards' them to the graveyard while the latter 'sends' them to the graveyard.
Another example is the effect of Dark End Dragon. Because its effect sends monsters to the graveyard, it cannot be chained to by the effects of Stardust Dragon, My Body as a Shield, Destruction Jammer, and the like because it does not 'destroy' the monster.
Yet another is why the effects of Sangan, Dandylion and The White Stone of Legend activate when they are tributed. Their card texts state 'When this card is sent (from the field) to the graveyard...' Therefore they do not have to be destroyed unlike Giant Rat, Shining Angel, etc.