IS PYTHON a Replacement for a Linux Shell? Is it a SHell language?

As Mehdi Gholam stated, it is a programming language and it can be used as shell, but it is quite inconvenient as such. It can be a shell because it has an interactive interface, but it is inconvenient because the main shell commands you normally use in a shell are method calls, it won't let you use pipes and other process related things easily, and because the syntax of the language is not adequate for writing short commands.
But there is an other aspect: you can use it efficiently as shell script language, as you don't need to use the same shell for the command line and for scripting. Just some examples: http://www.linuxjournal.com/content/python-scripts-replacement-bash-utility-scripts[^], http://www.jperla.com/blog/post/a-clean-python-shell-script[^]

Python is not a shell. It is programming language. You can program many exploits using this programming language. So, the exploits that will spawn control over remote computers are also called as "SHELLS". Like this you can program exploits using perl, ruby, php like programming languages... Even if the exploits are called as "SHELLS", it is not original shell. In Linux the shells are "Bourne Shell(sh), Bash, C Shell(csh), K Shell(ksh), etc".

Also note it, python's commandline interpreter(open terminal/command prompt and run command "python", you can see python shell to run python commands/codes) is also known as "Python Shell".