The shell account is your way of talking to ice and telling it what you want to do. You will need a telnet program on your computer in order to access your shell account. While using your shell account, you will, of course, need to be connected to the Internet. You can tell ice to continue running programs and processing tasks when you log off, though.
If you have used MS-DOS to type commands, you will have some idea of the interface to the shell account. But why is it called a shell account? Let's split the term into two:
Shell: This term denotes a user-interface which shields you from the "innards" of the computer. The Windows95 shell consists of windows, icons, menus and is highly mouse-driven. Unix and MS-DOS is primarilly command-line driven (in other words, text-based). Ice uses the Unix operating system, and therefore Unix shells. The plural shells is used because there are different ones available to try out - they allow you to put commands together differently and have different editing features. The one you are set up to use by default is called BASH (the Bourne Again Shell), which has several useful facilities for making command entry easier.
Account: Don't worry - you don't have to pay anything! This simply denotes the fact that you have been given a username and password and other useful bits and pieces on the server. Through the username and password, you have the right to browse around the machine (although not secure bits of it, of course), save your data and so forth, via the command shell.
Because Unix is a multi-user operating system, more than one person can be logged in at one time, through remote terminal connections to the server. A terminal used to be a keyboard and monitor (without much else) whose job is simply to present you with text-based to the server over a network. Today, you can get terminal emulators on your computer, which allow you to access the remote server over the Internet. The Internet's terminal emulation software is called Telnet. If you're using Windows95, Telnet is already installed on your machine, although it is well hidden (see below), but you might want to download better Telnet software from Tucows.