An Imprimatur is an official declaration from the hierarchy of the Roman Catholic Church that a literary or similar work is free from error in matters of Roman Catholic doctrine and morals, and hence acceptable reading for faithful Roman Catholics.
It is of greatest significance in works directly addressing Roman Catholic theology and doctrine, and was introduced as a measure to reduce exposure, particularly of the laity, to heresy. The presence of the imprimatur was at one time a matter of the greatest concern to many Roman Catholics. (In fact, in some officially Roman Catholic countries, nothing could be legally published without such an imprimatur. This was a form of prior restraint or censorship.) Today it is likely of concern only to more orthodox Roman Catholics.
A Roman Catholic imprimatur can consist of up to three stamps, each followed by a signature (name and title):
These "stamps" and "signatures" are simply rendered in plain type on a page at the front of the book (i.e. they are not literal stamps and hand-written signatures), and are often followed by the date and place of signing, as on legal documents.
Following this, some works may also include the following statement:
"The Nihil Obstat and Imprimatur are official declarations that a book or pamphlet is free of doctrinal or moral error. No implication is contained therein that those who have granted the Nihil Obstat and the Imprimatur agree with the content, opinions or statements expressed."
(While at first glance this statement might seem contradictory, an example might be that of a Roman Catholic work that offered parenting advice -- the advice may not be morally wrong or contradict Roman Catholic doctrine, but it might not reflect the views on parenting of the censor or bishop.)
Imprimaturs are not automatically transferrable to later versions of a work. Any new edition also requires a new imprimatur to be obtained.
The imprimatur can be revoked if, upon further examination, any doctrinal or moral error is found to be contained in the work.

Imprimi potest (Latin, meaning "able to be printed") -- If the work is that of a member of a religious order, this stamp indicates that it has first been examined and approved by the religious superior or head of the religious order (or a duly appointed representative).
Nihil obstat (Latin, meaning "nothing hinders") -- This stamp indicates that the work has been examined and approved by the censor of the diocese, and that he finds it free of doctrinal or moral error. The censor is often a scholarly priest appointed by the bishop, and it is his task to work back-and-forth with the author of the work to correct any inaccuracies or problems.
Imprimatur (Latin, meaning "let it be printed") -- Finally, this stamp indicates that the work has been approved for printing by the bishop of the diocese, or other ecclesiastical authority.