A couch is a sofa — an upholstered piece of furniture that seats more than one person. It's nice to have a couch that's big enough for your whole family to sit on while you watch TV.
The word couch comes from the French word coucher, meaning to lie down. Some types of couches are indeed meant for lying down. A psychiatrist's couch, for example, was designed to make patients feel comfortable when discussing their problems. In language, to couch is to phrase your words in a certain manner. If you don't want to hurt your friend's feelings, you should couch your words carefully when you tell her that her everyone is leaving her party early.