A profile is a building block for any Gentoo system that allows you, the administrator, to build your system with a range of default packages that suit a given purpose.  For example, the '/usr/portage/profiles/default/linux/x86/2008.0/desktop' profile sets some default USE flags which are best suited for a desktop environment.  Likewise for the 'developer' and 'server' profiles.  If you are browsing, you might notice that the contents of the 'parent' files in these directories point to the directory which contains the files that define that particular profile.