The Lemonade Stand is seen in the episode Karate Star. Harold owns it. Patrick karate chopped it in half. In the episode Krabs a la Mode, lemonade is seen and mentioned when SpongeBob gives Mr. Krabs a glass of lemonade, although it is highly unlikely he got it from the Lemonade Stand.
The Lemonade Stand only appears in the episode Karate Star. Harold was selling lemonade when Patrick karate chopped it in half. It is unknown if it will appear in other episodes.
It is a large hut with a picture of a lemon on top. Harold works in it. It sells lemonade (obviously), and it probably tastes good.