There are two types of tequila.
Mixto: 51% of sugar came from agave, the other 49% of sugar came from a different cheap type of sugar, usually cane sugar. That will influence the final taste, value and quality of the tequila.
100% agave: 100% of sugars comes from agave. It creates a superior product.
Not until recently, it is that 100% agave tequilas have started gaining popularity over the mixto in the US. Whisky drinkers are being turned into tequilas since they are rested in bourbon barrels.