What are free radicals?

This is an explanation I had about cancer that includes free radicals, is it right?

• The explanation goes as following: "As a person ages, he has more free radicals in cells (stand-alone molecules traveling through the cell). Sometimes, free radicals hit the cells DNA and changes the DNA as a result. Most changes in DNA are inconsequential, but, sometimes, the DNA reprograms itself in such away that it multiplies and attacks otherwise good behaving cells. This state is called cancer." As such, cancer would be programmed in us and there is nothing we could ever do to eradicate it. Individually, one person that respects a good diet may increase its chances of avoiding cancer by minimizing free radicals (I heard that tomatoes are pretty good to that regard).

• Answer:

  In short, it is incorrect. It is overly simple. The body has some fairly aggressive ways of dealing with dead or dying cells: it has to eat (phage) them, and very often uses single or multiple electron oxidation to tear them apart. In addition, formation of peroxides, radicals, and oxidizing species are a natural part of (particularly muscle) signaling. This needs to happen. The "free radical theory" of aging and cancer has not been useful in making progress against cancer, and most serious research has moved on to other hypotheses.