To differentiate is to identify the differences between things, to discriminate among them. For example, if the light is dim at the party, you might find it hard to differentiate between the spicy bean dip and the chocolate sauce.
You can see different in differentiate. This will help you remember that it has to do with finding how things are different, or making them different. For example, identical twins look alike but if you get to know them, personality traits, speech patterns and their likes and dislikes help differentiate one from another. Also, when you wear a team uniform the number on your back helps coaches differentiate between teammates when everyone is playing fast.