What is it?
A Challenge checker uses program code that checks if a user has fullfilled the requirements of a challenge cache. Per the guidelines of Geocaching.com a challenge cache published after April 21, 2015, must include a link to a web-based challenge checker.  So far Project-GC is the only allowed challenge checker.
How does it work?
A challenge checker consists of two parts: a challenge checker script and a tag. The script is a piece of program code written in the programming language Lua. The script checks for a certain condition, for example a script can check if the user has found a cache on 365 calendar days. A tag associates one or more scripts with a specific challenge cache, and defines (customizes) things like cache type, quantity, region or what is needed to make a script match the challenge cache.
The tag links a checker script, a challenge cache, and (optionally) a set of parameters for the checker script. This means that the same script can be used both for a challenge that requires the user to have found 100 mystery caches and a challenge that requires the user to have found 1000 traditional caches, since the cache type and quantity of finds required is specified in the tag.
The same checker script can be used for multiple challenge caches, for instance, most calendar challenges are handled by the same checker script. The tag is what makes this possible.
Project-GC also calculates how difficult a challenge is, more information at: Challenge difficulty
Challenge checkers are created by volunteers and can be requested in the Checker request forums, make sure to read the READ FIRST-post before posting a request.
Visit Project-GC's FAQ to find answers to many of the most common questions.
Related paid membership feature
With a paid membership there is no limit on how many challenge checkers you can run. Non paying users are only allowed to run 10 challenge checkers per 24 hours. The limit is a rolling rate limit.
The checker script system can be used to create different tools to check for statistics that are not otherwise available at the site. See Tools in the checker system for more information.
By default all geocaches with the Challenge attribute will be considered as challenge caches. Exceptions to that rule can be added here Challenge Exceptions Either by adding additional challenges, or by adding geocaches that shouldn't be listed as challenges. A group of cachers who have moderation rights vote on any caches that appear on the list. A moderation team have been voting on caches for several years but when the introduction of the 'challenge cache attribute' was introduced it had been hoped that correct use of that would eliminate the need for moderation - however it soon became evident that moderation was still required and so this exception list continues to be used.
Though this list can be viewed by all, only the moderators have access to vote - The list is sorted on a confidence score, the more homogeneous votes, the higher the confidence. The order in the list is only updated on page load. Use the up-arrow to vote that it is a challenge, and the down-arrow if it's not a challenge. The following was advice for moderators regarding voting consideration - Note that your vote shall not be based on today's Challenge guidelines but rather on the fact that the Geocache requires you to do something before/when logging it. You shall not take into consideration if it's a good challenge or not, neither if you like it as a challenge.
Only geocaches with a confidence score of at least 35% will be be considered as an exception. Three homogeneous votes should be enough for that, while for example 3 up-votes and 1 down-vote won't.
There is a forum available to discuss special cases and believes related to these exceptions. Checker Exceptions Forum The forum is readable by everyone, but only those with voting access can write in it.
- Guidelines for Challenge caches at Geocaching.com's Help Center.