Here's a 4-page overview of its support for web services. Note the Web services controller on page 4.
Since its PHP based you can use one of the following:
- The phpxmlrpc library(used by Joomla) OR the IXR library (used by Wordpress)
- Then Read on the xmlrpc specification
- Learn best practises from looking at the Joomla & Wordpress implementations. Security is very important
All the best!
EDIT: There's also the built-in PHP library that you can enable as an extension