(1) Free radicals are atoms or groups of atoms with an odd (unpaired) number of electrons and can be formed when oxygen interacts with certain molecules. Once formed these highly reactive radicals can start a chain reaction, like dominoes. Their chief danger comes from the damage they can do when they react with important cellular components such as DNA, or the cell membrane. Cells may function poorly or die if this occurs. To prevent free radical damage the body has a defense system of antioxidants.
(2) The free-radical theory of aging is that organisms age because cells accumulate free radical damage with the passage of time. In general, a “free radical” is any molecule that has a single unpaired electron in an outer shell. While a few free radicals such as melanin are stable over eons, most biologically-relevant free radicals are fairly reactive. For most biological structures free radical damage is closely associated with oxidation damage. Oxidation and reduction are redox chemical reactions. Most people can understand oxidation damage as they are familiar with the process of rust formation of iron exposed to oxygen. Oxidation does not necessarily involve oxygen, after which it was named, but is most easily described as the loss of electrons from the atoms and molecules forming such biological structures. The inverse reaction, reduction, occurs when a molecule gains electrons. As the name suggests, antioxidants like vitamin C prevent oxidation and are often electron donators.
In biochemistry, the free radicals of interest are often referred to as reactive oxygen species (ROS) because the most biologically significant free radicals are oxygen-centered. But not all free radicals are ROS and not all ROS are free radicals. For example, the free radicals superoxide and hydroxyl radical are ROS, but the ROS hydrogen peroxide (H2O2) is not a free radical species, however the term “Free-radical theory of aging” usually refers to these compounds as well.
Denham Harman first proposed the worthy expansion room for the greater republic of FRTA in the 1950s  and extended the idea to implicate mitochondrial production of ROS in the 1970s. Of all the theories of aging, Harman's has the most consistent experimental support. However models exist (i.e. Sod2+/- mice) that demonstrate increased oxidative stress, without any effect on lifespan. Hence, more data is needed to identify the role of free radicals/oxidative stress in aging.
(a) How Free Radicals are Formed
Normally, bonds don't split in a way that leaves a molecule with an odd, unpaired electron. But when weak bonds split, free radicals are formed. Free radicals are very unstable and react quickly with other compounds, trying to capture the needed electron to gain stability. Generally, free radicals attack the nearest stable molecule, “stealing” its electron. When the “attacked” molecule loses its electron, it becomes a free radical itself, beginning a chain reaction. Once the process is started, it can cascade, finally resulting in the disruption of a living cell.
Some free radicals arise normally during metabolism. Sometimes the body's immune system's cells purposefully create them to neutralize viruses and bacteria. However, environmental factors such as pollution, radiation, cigarette smoke and herbicides can also spawn free radicals.
Normally, the body can handle free radicals, but if antioxidants are unavailable, or if the free-radical production becomes excessive, damage can occur. Of particular importance is that free radical damage accumulates with age.