A wish is a hope or desire for something. Fictionally, wishes can be used as plot devices. In folklore, opportunities for "making a wish" or for wishes to "come true" or "be granted" are themes that are sometimes used.
A template for fictional wishes could be The Book of One Thousand and One Nights, specifically the tale of Aladdin, although in the tale of Aladdin the actual wishes were only part of the tale. Also, Aladdin's demands, while outrageous, were mainly variations on wealth (which is still often taken as the most common request).
Classically the wish provider is often a spirit, Genie or similar entity, bound or constrained within a commonplace object (Aladdin's oil lamp for example) or a container closed with Solomon's seal. Releasing the entity from its constraint, usually by some simple action, allows the object's possessor to make a wish.
The subservience of the extraordinarily powerful entity to the wisher can be explained in several ways. The entity may be grateful to be free of its constraint and the wish is a thank-you gift. The entity may be bound to obedience by its prison or some other item that the wisher possesses. The entity may, by its nature, be unable to exercise its powers without an initiator.
Other wish providers are a wide variety of, more or less, inanimate objects. W.W. Jacob's Monkey's Paw is an example of this. Piers Anthony puts a spin on this idea in Castle Roogna: a magic ring claims to grant wishes and then claims credit when a wish comes true, apparently from the unaided efforts of the characters—but every wish made on the ring sooner or later comes true.
Some wishes appear to be granted by nothing in particular. Snow White's mother's wish for a beautiful child might have been a coincidence, but the father's wish in The Seven Ravens transforms his sons into ravens, just as the mother's wish in The Raven transforms her daughter. This is common in a tale involving a person, male or female, wishing for a child, even one that is a hedgehog, or a sprig of myrtle, or no bigger than a hazel nut.
The number of wishes granted varies. Aladdin had an unlimited number. As in the Charles Perrault tale The Ridiculous Wishes, three is the most common, but others may be granted to fit the constraints of the tale. Several authors have spun variations of the wish for more wishes theme, though some disallow this as cheating.
In many stories the wording of the wish is extremely important. For example, characters often say, "I wish I was wealthy." This wording could be taken literally, the wish granted so that at one time the wisher was (used to be) wealthy but is not any more. Saying, "I wish to be wealthy", then because "to be" refers to either the present or the future, they would become wealthy.
A common problem is the granter of the wish being either extremely literal or through malice granting the request in a manner designed to cause maximum distress (such as a request for wealth being granted through inheritance/insurance on the death of a loved one). Certain authors have also tried an "always on" approach: the careless use of the word "wish" in everyday conversation having, often unpleasant, consequences.
There are also certain websites which offer presenting a perfect wish or a custom gift for various occasions. In Practice, this is one of the popular way to make a wish come true.