In simple terms an art print is a reproduction of an original painting. There are numerous methods of printing and a large variety of different mediums that the print can be reproduced on. The majority of art prints are reproduced using a method know as Giclée.

Most often associated with reproductions a giclée is a multiple print or exact copy of an original work of art that was created by conventional means (painting, drawing, etc.) and then reproduced digitally, typically via inkjet printing.
In giclée printing, no screen or other mechanical devices are used and therefore there is no visible dot screen pattern. The image has all the tonalities and hues of the original painting. With most giclée printers now having 8 - 12 colour process the reproduction quality is as close to the artists original as is possible.
Using this method the reproduction can be produced onto a large variety of different mediums, from smooth graphic paper, photographic paper, textured paper and even canvas.
Limited edition prints are traditionally signed and numbered in pencil with the edition number on the bottom left, the title in the middle and signature on the right. It is generally accepted that the publisher can mark A/P (Artists Proof) on up to ten per cent of the edition - so an edition of 100 would have numbers 1/100 - 100/100 and an extra ten marked A/P. The size of the edition can depend on a number of factors but the trend now days is to keep the edition size fairly low to increase credibility.
The edition would normally be administered by the publisher, insuring each print is numbered sequentially until the stated edition size is reached. Where possible the limited edition print is signed by the artist and is supplied with a certificate of authenticity which states the edition size and number of that particular print. 
The obvious answer to this question is that an original painting is one of a kind, and when you purchase an original, you purchase the only one in existence. Of course there could be reproductions of the original, either open or limited edition. But the original is the one the artist created.
Many people like knowing that they own the original painting, and they are willing to pay the price for this privilege. Other people, however, wish to collect fine art on their walls, but they do not have the budget to pay for an original. For these collectors, a print is an affordable way to enjoy the unique work of a particular artist.