X-terminal Emulation in Windows
Unix computers use a protocol called X11 to provide windows-like functionality in Unix applications. This interface is commonly called a graphical user interface or GUI interface. Most Unix applications can be invoked by the user in either an X11 mode (GUI mode using an X-terminal) or a text-based mode that does not use the GUI. Text-based versions of applications will not render graphics, nor will they have the familiar pull-down menus associated with common MS windows applications.
Often users will want to invoke Unix applications from PCs running MS-windows. For simple text-based applications, this is not a problem and can be accomplished using a terminal window in putty, etc. However, users wishing to run Unix applications with GUI interfaces must install emulation software on the PC. Emulation software allows Unix GUI applications to be rendered in an MS windows environment.
Note that Linux and MAC OS-X users are immune from such concerns, as these operating systems have built-in X-terminal emulation (i.e., these operating systems are Unix-like).
Free Xterm emulators currently available for Windows include: