A good question, sharksfan11 - although,
of course, there are no bad questions

A reprint is made with the original plate, and usually by the proper authorities. It may be valid for postage, or it may have been made purely for sale to collectors.
Here is an example from Jammu & Kashmir. The pair of ˝ Anna stamps at left are originals; the single at right is a reprint:

How do I know it's a reprint? It's too well printed

The originals were always messy, like the pair. When the Jammu & Kashmir post office decided to reprint these to sell to collectors around 1880, they took great care to see that the stamps were clearly printed, on better quality paper ...
And here is a forgery of the same stamp:

Don't worry about the difference in colour. The ˝ Anna stamp was quite legitimately issued in red as well. This stamp was made in imitation of the real thing, and fooled collectors for some years. These forgeries were included in official supplies sent from Jammu & Kashmir - and the paper, ink and printing all look like the real thing. The design has subtle differences though. (Hint: compare the left-most curved stroke in the central circle).
It appears that some crooked staff of the post office decided to make some money by creating their own stamps, inserting their fakes into the official supplies, and removing and selling the genuine stamps. They got away with it for a long time, too
