Evaporated milk is a shelf-stable milk with 60% less water (thus, 60% less volume) than regular milk. Stabilizers and vitamins are added, but no sweeteners or flavorings. It's often used in savory baking recipes (like bread); if a dessert recipe calls for evaporated milk, it will call for some kind of sweetener as well.

Sweetened condensed milk, on the other hand, is evaporated milk plus sugar. After 60% of the water is removed from the milk, sugar is added to create a thick liquid that's super sweet and more like syrup than milk.

IN A NUTSHELL: Sweetened condensed milk is evaporated milk with sugar added.

Can I substitute one for the other? No. Evaporated milk is thin and savory, sweetened condensed milk is thick and syrupy sweet.