Fasting -- The law of fasting requires a Catholic to reduce the amount of food eaten from normal. The Church defines this as one meal a day, and two smaller meals which if added together would not exceed the main meal in quantity. Such fasting is obligatory on Ash Wednesday and Good Friday. The fast is broken by eating between meals and by drinks which could be considered food (milk shakes, but not milk).
Abstinence -- The law of abstinence requires a Catholic to abstain from eating meat on Fridays during Lent in honor of the Passion of Jesus on Good Friday. (Meat is considered to be the flesh and organs of mammals and fowl.)