XML-RPC is a remote procedure call (RPC) protocol which uses XML to encode its calls and HTTP as a transport mechanism.
XML-RPC is a means of encoding RPC requests in XML and sending them over HTTP. The use of XML as the intermediate language allows nesting of parameters into maps and lists, thus larger structures can be transported. Therefore XML-RPC can be used to transport objects or structures both as input and as output parameters.
In comparison to REST, where resource representations (documents) are transferred, XML-RPC is designed to call methods. You can find the homepage here.