zsync is a file transfer program. It allows you to download a file from
a remote web server, where you have a copy of an older version of the
file on your computer already. zsync downloads only the new parts of
the file. It uses the same algorithm as rsync.

zsync does not require any special server software or a shell account
on the remote system (rsync, in comparison, requires that you have an
rsh or ssh account, or that the remote system runs rsyncd). Instead, it
uses a control file - a .zsync file - that describes the file to be
downloaded and enables zsync to work out which blocks it needs. This
file can be created by the admin of the web server hosting the
download, and placed alongside the file to download - it is generated
once, then any downloaders with zsync can use it. Alternatively, anyone
can download the file, make a .zsync and provide it to other users.
