OpenVPN is a VPN service that allows remote networks or wireless clients such as laptops, to connect to IPFire. This functionality is also available with the implementation of IPsec, but OpenVPN takes a different approach, based on SSL tunnels.
Using OpenVPN instead of IPSec is a matter of preference, though there are a few very good reasons to choose one over the other.
See this article for some additional in-depth information about the two.
This documentation is divided into four areas. At first, everything worth knowing about is the configuration, for advanced users there are tips and tricks under extensions. Smartphones are often not so easy to configure, but therefor there are some help findable in this wiki.