For US Comics various terms and abbreviations are often used in the product names or descriptions. If you are new to the hobby, or if you've perhaps always wondered What does it mean?, you might find your answer here.
PTG is the abbreviation of "Printing" specifying the publication's print run. Most often this is used with comics which are being reprinted due to the previous print run being sold out and demand still being high. This would then result in a 2ND PTG (second printing), 3RD PTG (third printing) and so on. The cover of a reprint is usually changed (different coloring or new art altogether) to clearly indicate it not being a "first printing" but the contents of the publication are still the same (unless otherwise indicated).
VAR = Variant Cover
Indicates a different edition of a title that contains a cover image different from its original edition. For example, a comic is published with a cover by Artist X, but the publisher offers a variant featuring a special cover image by Artist Y as well. Variants can be offered alongside regular editions at regular pricing when a title is solicited ("free to order variant"), or can have an order qualifier ("incentive variant"). For example, a publisher may offer the retailer an opportunity to receive or purchase one variant edition of a book for every twenty-five regular copies ordered of that book (which would then be indicated as an 1:25 VAR). These incentive variants tend to have varying prices due to their scarcity.
(MR) = Mature Readers
This comic or publication is intended for Mature Readers and contains themes, language or content which might not be suitable for young children.
(A) = Adult
This comic or publication is intended for Adult Readers and contains themes language or content which is intended for Adults only.
HC = Hardcover
This graphic novel or publication features a hardcover.
TP = Trade Paperback
This graphic novel or publication features a softcover (sometimes also referred to as SC).
GN or OGN = Original Graphic Novel
Usually a Graphic Novel is a collection of several comics, but sometimes a story is immediately published in graphic novel format. This is then considered an "Original" Graphic Novel.
Virgin or Undressed covers
A comic or publication will usually have the title, issue number, publisher logos, names of the creative team, etc. on the front cover. A "Virgin" or "Undressed" cover indicates that all these things have been removed so that the artwork is fully visible.
SGN is the abbreviation of "Signed" indicating that this publication will be signed by one or more people of its creative team. These items usually have a strict and limited print run.