R is an open source statistical software. ( actually not...."Many users think of R as a statistics system. We prefer to think of it of an environment within which statistical techniques are implemented"- the R project). In this blog, I am discussing it as statistical software.
It is basically based on command language interface (not Point and Click). although it has some basic GUI and a GUI package called R commander.
This is based on S language. S plus is commercial version and R is open source (GNU). R is very powerful software because of its flexibility. You can get almost what you want as you can parameterize the functions yourself. This is very good in graphics.
Where it runs?
It runs in Unix/ Linux based computers, windows and Mac.
Where to get it?
You can get the software from various mirror sites going through http://www.r-project.org/.
It will come with some built in functions and you can always add some packages according to your requirements.