Xml-rpc is an old but widely used protocol for webservices in xml/json, wich evolved in what is now soap, the protocol works over http (like web pages) and basically uses xml to encode it's calls/responses.
In this scheme (stolen by this article form joomla.fr, many thanks to Garstud) you can see an overview of Joomla/Joooid communications :
This scheme is the Joomla 1.5 way to handle xml-rpc webservices, for Joomla 2.5 the mechanism is slightly different because xml-rpc are not in Joomla core anymore.
Here the structure of object Category, as it returns from the server:and this is the structure of object Article
Joooid Rpc for Android is an Android Library containing the rpc-client used in Joooid App, including debugging options. Joooid Rpc for Android is based on android-xmlrpc project and is a wrapper for the Joooid api exposed by Joooid Component/Plugin for Joomla 1.5 and 2.5, you can use them in any Android project simply adding the Joooid Library Project to your project (tutorial : how to reference an android library).
The following snippet of code is an example of use of Joooid Rpc for Android :