One of the challenges of open source is keeping up to date with the more frequent patches and security fixes. Someone on your team will have to know how to do these.
Joomla.org releases patches which are a bunch of files that need to be applied/ftp'ed onto your server.
Major releases come around every year or so with many new features. The current version is 1.5 and 1.6 will be released in the next 12 months.
There are some extensions and services that will keep you site updated for you.
- Official Joomla project download files
- Official Joomla security center
- Managed Joomla Hosting (simplweb.com)
- An Update Manager for Joomla!
- Upgrading 1.5 from an existing 1.5x version - (Joomla! Documentation site)
- Update Joomla in under 30 seconds (compassdesigns.net tutorial)