It's just a Web-browser (Firefox), advanced e-mail (Thunderbird) and newsgroup client, IRC chat client, and HTML editing software Internet suite; all your Internet needs in one application.
The old Mozilla Application Suite in a new package; if u dunno.
Same as having Firefox, Thunderbird & their ilk installed separately.
Download & install only if u frequently use an emailing application a la the Outlook Express & also edit your webpages (Microsoft Frontpage would do the same job)
Otherwise... just install Firefox.. it gets updated more frequently & is lighter.