The built-in WebDAV server allows you to easily transfer files to Textastic from your Mac or PC via Wi-Fi.
Windows and Mac OS X include WebDAV clients so you can use Windows Explorer and Finder to manage the files in Textastic.
Textastic is tested against Mac OS X Finder, Windows Explorer (Vista and Windows 7), Cyberduck, BitKinex and the Total Commander WebDAV plugin.
If you want to know more about WebDAV, consult the excellent Wikipedia article on WebDAV.
Textastic 2.2 also includes a WebDAV client in addition to the WebDAV server.