From the website:
Terrier 2.1 is available for download. This is a minor update, which contains some bug fixes, and some minor improvements. Support for indexing various test collection has been improved (CLEF, TREC Legal track), and the flexibility of the settings of some applications such as the Desktop search and the Interactive Terrier have been enhanced.. Moreover, Terrier 2.1 includes a file system abstraction layer, which allows various types of files to be accessed through a uniform API. For example, indexing an HTTP Web page is as straightforward as indexing a local document. Moreover, a notable indexing bug affecting only the Windows platform was resolved.
For more detail, see the What's New page.