An action figure is a posable plastic figurine of an action hero, superhero, or a character from a movie or television program. These dolls usually are marketed as merchandise intended for boys. - TheFreeDictionary.com
http://encyclopedia.thefreedictionary.com/action%20figure

Introduced by Hasbro in 1964, GI Joe is considered the granddaddy of all action figures, and was, on November 11, 2004, honored with induction into the National Toy Hall of Fame (http://www.strongmuseum.org/NTHoF/gijoeframeset.html).

Marketed to appeal to boys who wouldn't be caught dead with a Barbie doll, it was comparable in size (11 1/2") and was similarly "accessorized" with changes of uniform and equipment. It was immediately successful, but anti-war sentiments in the ensuing years led to GI Joe pursuing more non-military activities as an astronaut, Indiana Jones-type archaeologist, and big-game hunter. Interest declined, along with the size of GI Joe (an 8" figure was released in 1977), and the concurrent oil crisis gave Hasbro an excuse to terminate production in 1978. (Plastic is a petroleum-based product.)

Inspired by the success of Star Wars merchandise, the product line was revived in 1982 with the introduction of 3 3/4" figures. Interest began to revive, and the 12"-size figure was re-introduced in 1991, new lines of 4 1/2" and 6" figures were manufactured in 1995 and 1996 respectively, and the entire 1964 product line was reissued in 2004 to celebrate GI Joe's 40th anniversary.

As with any collectible, condition and rarity are the deciding factors when considering value. Mint in original packaging is most desirable, with seriously damaged pieces being good only for parts. The GI Nurse Action Girl issued in 1967 is so rare (what boy worth his salt would have bought a girl doll!?) that MIB (mint in box) specimens can command $6000+ on the secondary market. And at a recent online auction, a 1967 GI Joe Fighter Pilot Outfit (sans figure!) realized a final bid of $6362.62! This item was purportedly found at a flea market and still bore its original price of $5.99.

Thousands of GI Joe products have been marketed over the years, and with all the different issues and issue variations, not to mention fakes, mistakes, and prototypes, it would be wise to refer to print and online guides to sort them all out. To that end, the following are recommended:

Books

"Collectible GI Joe: An Official Guide to His Action-Packed World," by Derryl Depriest
http://www.amazon.com/exec/obidos/ASIN/0762405368/auctionbytescom
Considered by some to be the "bible" for this collectible.

"GI Joe: The Complete Story of America's Favorite Man of Action," by John Michlig
http://www.amazon.com/exec/obidos/ASIN/0811818225/auctionbytescom Highly regarded history of the popular collectible.

"GI Joe: Official Identification and Price Guide 1964-1999," By Vincent Santelmo
http://www.amazon.com/exec/obidos/ASIN/0873417798/auctionbytescom

Websites

Action Figure Checklist
http://www.actionfigurechecklist.com
Illustrated database (1982-present) for keeping track of your collection.

The Adventure Team Headquarters
http://www.nyaj.com/newat.html
Comprehensive "visual reference guide" to the GI Joe Adventure Team product line issued from 1970 to 1976.

Complete Guide to G. I. Joe: A Real American Hero
http://joes.propadeutic.com/index.html
Lots of info, including: how to date different versions, mechanical repairs, chronological lists, links, more.

Figures.com
http://www.figures.com
General site devoted to action-figure news

TheFreeDictionary.com
http://encyclopedia.thefreedictionary.com/GI%20Joe
(TheFreeDictionary.com is also an encyclopiedia.) Succinct history of the GI Joe action figure, with links.

GI Joe Action Figures
http://www.hasbro.com/gijoe/default.cfm

Hasbro's official GI Joe website
http://www.hasbro.com/gijoe/pl/page.history/yr.60/dn/authentic/default.cfm
History of GI Joe

The Official Hasbro G.I. Joe Collectors' Club
http://www.gijoeclub.com
Monthly newsletter, free monthly ad, limited edition merchandise.

Yo Joe!.com
http://www.yojoe.com
Major site with lots of information on: identifying figures by date stamp, prototypes, identifying accessories by color, more. Great links. Online magazine.