Although the term 'hailstorm' is used in common speech to indicate that hail is falling from a storm, there is actually no storm which produces nothing but hail. Therefore, a hailstorm doesn't actually exist in the sense that a rainstorm or snowstorm does. Instead, a fairly limited area of a regular storm, known as a hail shaft, is the place where a fall of hail occurs.
Hail is a very dramatic weather event and can cause a lot of damage on the ground if the hailstones are large enough. A hailstone forms in very powerful updrafts inside thunderstorms, as ice condenses out and is thrown aloft multiple times by updraft action. The updrafts must be very strong to keep the ice aloft long enough to build up to the size needed to qualify as hail. Hail can range from two tenths of an inch to over six inches, depending on how strong the updrafts are that suspend it in the cloud.
The more violent the thunderstorm, the larger the hail it produces is likely to be. Tornadic thunderstorms produce the largest hail, more often than not, and the observed size of hail can sometimes be a rough guide to how likely a tornado is in the vicinity in the near future.
Eventually, after formation, a mass of hail will fall, often carried to the earth by a strong downdraft. This downdraft is called a hail shaft, and the track of hail it leaves on the ground is known as a hail streak or hailstreak. Depending on how much hail has formed, a fall of hail can last anywhere from a few minutes to a quarter of an hour. Some freak hailstreaks persist quite a bit longer and can inflict large amounts of damage.
Any hail that is an inch wide or larger has the potential to cause damage on the ground. Plants are especially vulnerable to hail, since their leaves are thin and easily torn, and they obviously cannot move to shelter when hail begins to fall. Whole fields of crops may be flattened by a hailstorm, decorative plants can be damaged or destroyed, and trees and other natural plant life injured as well. Hail can be large enough to injure people and animals, especially when shelter isn't readily available. Some people have been killed by hail, although this is fairly rare, since exceptionally large hailstones are needed to cause enough impact on the head to damage the brain.
Besides crop damage, hail can also damage vehicles by denting their thin metal, or breaking windshields. Skylights can also break under the impact of hail. Fortunately, large hailstones usually don't vary from a vertical fall very much, since they are heavy and fast-moving, but if they do, they can crack or shatter windowpanes in houses.
Hail usually occurs in summer, which is when thunderstorms with powerful updrafts occur. Thundersnow -- thunderstorms in winter -- often lack updrafts strong enough to produce hail, although on rare occasions, one will be powerful enough to produce small hail. Hail can sometimes accumulate to a depth of several inches in the summer along a hailstreak, however, bringing a touch of wintry whiteness to the warmer season of the year.