Android is an operating system, designed primarily for mobile phones and devices but will see some use in ultra portable laptops (netbooks). It was developed largely by Google and the Open Handset Alliance. While recent releases allow "native code" to be developed, most applications are designed using Java. More details on Android and it's history are available at the Android (operating system) page at Wikipedia.
Geocaching Applications on Android
There are several Geocaching applications available on Android, including:
- Groundspeak's Geocaching for Android ($9.99) 
- GeoBeagle (FREE) 
- GeOrg (€4.99) 
- Find Geocaches ($1.99)  (requires GeoBeagle, OpenGPX, or GeoHunter)
- NeonGeo (€2.99)  (uses Geocaching Live, Groundspeak's public API)
- GeoHunter (FREE)  (based on GeoBeagle source)
- CacheMate ($4.99) 
- Geocacher ($14.99) 
- Geodroid ($7.99) 
- GeoFun ($6.99) 
There are also many compass and GPS related programs (such as GPS Status )available on the Market.
The above lists is not all inclusive and is presented in alphabetical order. It is presented as a way for Android users to find GeoCaching applications, and their Cacheopedia page if they have one. The links following the applications are to the official page for the application if it has one, otherwise to its Cyrket page.