In its most primitive form, it simply consists of in game characters communicating valuable game information via an out of game medium (e.g. this wiki, internet forums, or instant messaging/email).
What one person sees as legitimate metagaming another may view as unfair advantage or even cheating. If a metagaming action is particularly powerful, difficult to explain in character, or unusual, it is more likely to be viewed by some as unfair. Unsurprisingly, any complaints there are most often come from those not benefiting from the action.
Some view the exploitation of bugs and quirks advantageously to one's character as metagaming. Others view it as cheating. Often common acceptance of an exploit depends on developer reaction and the spirit of the game. There have been numerous cases where developers embraced bug exploits and revelled in the unexpected use of a game's "quirks" such as rocket-jumping in Quake or skiing in Tribes.
Another example that has proved controversial is the DEM's use of their DEMON information sharing tool, which allows characters running the tool to share information with one another. When first released, it made no checks to ensure that only DEM members were using the tool (e.g. an alt of a DEM character could use it to gain access to information from the DEM alt), and allegations that it was a form of multi-abuse were levied against it. A revised version of the tool was later released which addressed these allegations by automatically disabling itself for all non-DEM characters or DEM characters in zombie form.
That said, DEMON is by no means the only controversial tool or resource. Other examples include UDBrain, UD Automapper, and even the wiki itself, which is said to enable sharing of information between characters that should not be shared.
For those who seek full character immersion, the term "metagamer" may be considered an insult.