In general, new programs arrive on your Mac via one of two avenues: on a CD or DVD, or via an Internet download. The CD method is slightly simpler; see "Performing the Installation" later in this section.
For help installing downloaded programs, on the other hand, read on.
Programs you download from the Internet generally arrive in a specially encoded, compressed form. The downloaded file's name usually has one of these file name extensions:
.sit indicates a StuffIt file, the standard Macintosh file-compression format.
.zip is the standard Windows compression file format. And because Tiger has a built-in Create [.zip] Archive command right in the File menu (and doesn't come with StuffIt Expander), .zip is becoming the new standard Macintosh compression format. (It certainly makes life easier for people who have to exchange files with the Windows crowd.)
.tar is short for tape archive, an ancient Unix utility that combines (but doesn't compress) several files into a single icon, for simplicity in sending.
.gz is short for gzip, a standard Unix compression format.
.tar.gz or .tgz represents one compressed archive containing several files.
.dmg is a disk image, described below.
Fortunately, if you use Safari (Chapter 21) as your Web browser, you don't have to worry about all this, because it automatically unzips and unstuffs them. If you use some other browser, StuffIt Expander can turn all of them back into usable form when you download ...