Back in the old days, before personal computers appeared, people used terminals instead of personal computers. Many of the computer operating systems that were created back then are still with us. One of these is unix. There are many unix-like operating systems, for example: FreeBSD and Linux.
Unix is a very powerful system and since it is the system on which the Internet was developed, it integrates with the Internet better than any other. Almost all Web servers are running on unix systems.
For those that adopt unix as their main operating system, there is a network-transparent windowing system available. For those that are just visting it by by using its shell, unix expects such users to be using a terminal like they did in the old days.
When you log in to Idiom to get a unix shell, you must tell Idiom what kind of "terminal" you are using. Unfortunately, most terminal emulator programs don't emulate real terminals very well and so there are often glitches.
The word "shell" is a generic term for the command interpreters used on unix. Unlike DOS where the command prompt is part of the operating system, on unix the command prompt is just another program. There are many different such programs available.