Rough
The rough is the tall grass beyond the fairways.
The rough is the long grass to the left and right of the line of play. It is meant to make the mishit harder without making it impossible. Depending on the course it grows in stages: semi-rough directly at the fairway, behind it the actual rough, sometimes knee-high. On difficult courses and at tournaments it is deliberately left to stand high. Whoever plays a lot into the rough loses distance and control, because grass between clubface and ball makes the contact soft.
Practically that means: the ball sits deeper, flies shorter and can hardly be controlled with spin. An example: your tee shot lies in the dense rough, 150 metres from the green. A long iron is risky there because the grass catches the clubface. The safe choice is a wedge back onto the fairway. This decision saves more scores than any hero shot. Rule of thumb: the deeper the ball sits, the more loft you need.
One misunderstanding concerns the searching. You may search for your ball in the rough without penalty, also with your feet. If you do not find it within three minutes, it is deemed lost. If you move it accidentally while searching, since the current rules there is no penalty stroke any more, the ball is replaced. Still the rule holds: search attentively but quickly, otherwise the pace of play suffers.
FAQ
How do I best play my ball out of the rough?
The deeper the ball sits in the grass, the more loft you need. In dense rough with still long distance the long iron is risky because the grass catches the clubface — the safe choice is often a wedge back onto the fairway. This decision saves more scores than any hero shot.
Do I get a penalty stroke if I move my ball in the rough while searching?
No. Since the current rules there is no penalty stroke when you move your ball accidentally while searching — you simply replace it. You have three minutes of search time; after that the ball is deemed lost. Search briskly anyway so that pace of play does not suffer.
Why does the ball fly shorter and more unpredictable out of the rough?
Grass between clubface and ball makes the contact soft and takes away spin and energy. The ball therefore flies shorter, flatter and less controllable and can hardly be stopped on the green. That is exactly the point of the rough: it punishes the mishit without making it impossible.