Types of Caches
Geocaching basically involves treasure hunt. You get the location of a container filled with various items and then you go out there to find it. Then you log your visit and move on to the next one. I'd say it's an interesting experience in and by itself. However, there's never enough creativity. People have invented various types of the game to make it even more involving. Here are some descriptions of the most common variations played all around the world.
Traditional Cache
The original type of cache, basically including only the log book in the container. The container is made of some water proof material. The most commonly used are tupperware, bucket or ammo box.
A little more advanced traditional caches are filled with goodies, such as trinkets, coins, small toys etc.
The most important rule is to take something out of the cache and put something else into it. Just remember to read the description carefully before going hunting. While the coordinates are the exact location of your cache, there might be some additional clues as to what to put inside. Even the most basic caches can be themed.
Earthcache
Earthcaches are a source of information about Earth. They are placed in particular spots where you can learn about the geology of the planet, its geological processes, resource management and scientific methods of gathering the information. They usually include notes on where to find the location. They're a type of themed caches.
History cache
Another theme for traditional caches may be culture or history. The location of the cache is also the spot about which you can learn when you find your treasure. Educational notes, artifacts such as regional musical instruments or paintings by local artists are common.
Multi-Cache (offset Cache)
A multi-cache is a cache within a cache. When you find your first one – the one listed on the website - it should contain the coordinates to locate the next one. Sometimes this next one will lead you to another one and another one… You just get multiple adventures: two or three in one.
Letterbox Hybrid
This is a different type of cache. Instead of the exact coordinates, it only gives you clues as to where the cache is. Sometimes you can find both – a letterbox with coordinates and there's a geocache to be found. Another version of multiple-cache with a twist. The letterboxing hybrid has its own website as well.
Event Cache
This one is usually made as an archive of a meeting. Geocache players do have lots of forums and they communicate through caches but the local organizations sometimes have meetings to discuss things in person. Such caches are made to remind people of such meetings.
Mystery or puzzle caches
A very rich and growing category. The basic rule is that you don't get the exact coordinates of the cache's location. You only get some general reference. All you have to do is solve a complicated puzzle to get the idea about the real location. Since the game is still growing and more and more people are joining each day, the varieties are changing too. Personal creativity is actually the only limit to what kind of games within the game you can play.
Grandfathered Cache Types
These are cache types that are no longer available for creation on geocaching.com.
Virtual Cache
A virtual cache is the one that doesn't exist in the real world as a tangible object. Basically it's a task to be fulfilled, a question about the location, a puzzle to solve or an interesting spot to visit. Sometimes there is a logbook where you can log your finding but mostly you just go to the website and report your visit there. The idea is to visit the spot and share the information about it rather than collecting real world objects.
Due to the basic nature of these caches you really have to visit the spot and acquire the coordinates before you can post your findings to the website. In addition, such geocaches should be placed in particularly interesting places so that people find it worth to go and seek them.
Webcam Cache
Hunting for these caches requires cooperation with a friend. They use web cameras placed in public area such as parks, shops or roads. What you have to do is get yourself in front the particular camera and let it see you to log your visit. That's where you need a friend who stays home and looks up the camera website to record your presence. They then save the picture to log your visit.
Of course, you can also use a wireless modem to log yourself on the spot but I'd say it's only half the fun.
Locationless (Reverse) Cache
A very twisted variation of the game. Reverse cache is almost the opposite if the traditional one. What you do is not seek for a hidden object using it's coordinates but your task is to locate a specific object and prove your finding by reporting its location. It usually involves collecting waypoints of various objects all around the world.
This particular type of geocaching has evolved into a separate game called Waymarking. The basic rules are similar and there's a website and forums to chat. The difference is that locationless caches are categorized and you can search for the location in each category.