From the Bishops of the United States:
Ash Wednesday and Good Friday are obligatory days of fasting and abstinence for Catholics. In addition, Fridays during Lent are obligatory days of abstinence.
Fasting is obligatory from age 18 until age 59. When fasting, a person is permitted to eat one full meal, as well as two smaller meals that together are not equal to a full meal.
Abstinence from meat is binding upon members of the Latin Catholic Church from age 14 onward.
The distinction between fasting and abstaining is a little confusing, particularly because people have begun to talk about "fasting" from things like video games, television programs, etc. during Lent. Fasting was traditionally understood as voluntarily having an empty stomach for a while.
Abstinence, such as when one abstains from voting, means refraining from something. Abstaining from meat is refraining from eating meat, but not necessarily going with an empty stomach, since full meals may be eaten unless one is also fasting. Abstaining from alcohol, dessert, etc. is the typical "giving up" something for Lent tradition many Catholics still practice. These sacrifices typically have no relation to whether or not the person is also fasting.
Fasting is not enjoined on anyone whose age or medical condition makes it unwise. Abstinence is still a worthy practice during Lent. It strengthens our self-discipline as the opening prayer in today's mass acknowledges. In small matters, we experience victory over whims or passing pleasures strengthening our prudence and restraint. These virtues serve us well in everyday life and during times of temptation.