Shisha is what is heated and smoked when a hookah is being used. Shisha is typically nicotine and flavoring soaked in molasses. When the molasses are heated up, the flavoring and nicotine are released into smoke. There are many different types of shisha flavors - from Starbuzz's Blue Mist to Tangier's K-Peach - but each different type of shisha contains a different amount of nicotine.
The highest percentage of nicotine found in shisha is 5 percent, and the amount of nicotine is determined by if a flavor is washed or unwashed. However, the process of washing shisha is not public information (and shisha-producing companies do not want to share the knowledge.)
Shisha, pictured below, looks like seaweed. However, the coloring of shisha can range based on what is put in the shisha.