[RESTlos] (german for completely, totally) is a generic [Nagios] api. Generic means, it can be used with every core that understands the nagios configuration syntax (for example icinga, shinken, etc). It provides a RESTful api for generating any standard nagios object, modify it or delete it.
There are also some convenient functions for reloading the core (via command file) or verify the actual configuration via the REST interface.