Discovered attack vs double check
A double check is a discovered check, and a discovered check is a discovered attack. The three are nested, not parallel.
The difference in one sentence
A discovered attack moves one piece off a line so a piece behind it starts attacking something. If the uncovered attack hits the king it is a discovered check. If the piece that moved also gives check, it is a double check, and the king must move -- nothing can be captured or blocked.
The reason to separate them is how forcing each one is. A discovered attack creates two threats and the opponent gets to choose which to answer, so they will pick the cheaper one. A discovered check forces them to answer the check, which means the piece that moved is untouchable for a move and can go anywhere, including somewhere it could be captured.
A double check forces more than that: it removes every option except a king move. Capturing one checking piece leaves the other; blocking one line leaves the other. This makes it the single most forcing move available in chess, and it works no matter how much material the checking pieces are hanging.
That last point is worth stating plainly, because it is where most double checks get missed. In a double check, material is irrelevant. There is no legal move that takes either piece.
One of each
Discovered Attacks
Position 1
White to move. Find the best move.
Show the answer
32...Rf8 33.fxg4+ Kxg4 34.Rxf8
The line wins a rook. The piece that moves and the piece that attacks are different, so the defender has two problems and one move.
The position comes from an endgame of a real game, rated 635 for difficulty. The answer reaches a winning advantage.
Double Check
Position 2
Black to move. Find the best move.
Show the answer
22.b4 22...Ne2#
The line ends in checkmate. Two pieces give check at once, and no single move can answer both, so the king has to move whatever else is happening.
The position comes from the middlegame of a real game, rated 815 for difficulty. The answer reaches checkmate.
Questions people ask
What is the difference between a discovered attack and a double check?
A discovered attack uncovers an attack from a piece behind the one that moves. A double check is the special case where the moving piece gives check as well, so two pieces check at once and the king is obliged to move.
Can you block a double check?
No. Blocking stops one line and leaves the other, and capturing removes one checking piece and leaves the other. Only a king move is legal.
Why is a discovered check stronger than a discovered attack?
Because check must be answered immediately. That makes the piece that moved effectively untouchable for one move, so it can capture something defended, or move to a square where it would otherwise be lost.