Joven tequila is a style of tequila that takes standard blanco tequila and adds other ingredients to it in order to smooth or mellow the product. These tequilas are often called 'Gold' in the United States. Now, this can simply by a mixture of blanco and anejo tequilas, brought down to between 70 and 110 proof with water. Unfortunately, this name on a bottle of tequila can also mean that the blanco tequila has been colored with caramel coloring, flavored with oak extracts, and smoothed with glycerine and sugar syrup.
Most inexpensive bottlings of joven tequila are made through this addition of colorings and flavorings. Other tiers of tequila production ban this process - if you see reposado, anejo, or blanco on the label, it will not have these additions.