Bash is the shell, or command language interpreter, that will appear in the GNU operating system. Bash is an sh-compatible shell that incorporates useful features from the Korn shell (ksh) and C shell (csh). It is intended to conform to the IEEE POSIX P1003.2/ISO 9945.2 Shell and Tools standard. It offers functional improvements over sh for both programming and interactive use. In addition, most sh scripts can be run by Bash without modification.
DSH is a tool whose purpose is simply to execute a single command on a group of machines. For a simple use, you first need to setup a machines.list file (either in /etc or /etc/dsh) which is simply a file with one hostname by line.DSH will use either rsh, remsh or SSH to remotely execute the commands. Of course SSH is highly recommended as it is much more secure than the others. This means that you will have to deploy your SSH public keys beforehand, so that SSH can login on the remote host without interactively typing a password.
Screen is a terminal multiplexer that allows you to manage many processes through one physical terminal. Each process gets its own virtual window, and you can bounce between virtual windows interacting with each process. The processes managed by screen continue to run when their window is not active.
SED is a stream editor. A stream editor is used to perform basic text transformations on an input stream (a file or input from a pipeline)