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. (And unless you've changed the settings, they arrive in the Downloads folder stack on your Dock.)
The downloaded file's name usually has one of these filename extensions:
.sit indicates a StuffIt file, the standard Macintosh file-compression format of years gone by.
.zip is the standard Windows compression file format. And because Leopard has a built-in Compress command right in the File menu (and doesn't come with Stuff It Expander), .zip is 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 (Safari) as your Web browser, you don't have to worry about all this, because it automatically unzips and unstuffs them.
If you use some ...