In a balanced fitness program, strength exercise is important as well. You can call it resistance exercise because you are working against the muscles.
Resistance exercise can be done with weights or with rubber bands which you pull to extend and flex your muscles or with machines at the gym.

It is an exercise to build muscles, get more strength and prevent lose of muscles that comes with age. One of the greatest added benefits of strength exercise, especially for older people, is when the muscles get stronger, the person is more stable on his feet, thus can avoid falls and slips.