R is a general-purpose computer language. It is similar to S language, and much code written for S run unalterd under R. Its particular strength is in statistical, graphical capabilities. You can also do simple simulation with R. R is free: it's nothing to do with free as in beer. You get freedom to do whatever with R! So once you learn R, R will not disappear like a lot of other commercial products.
Some users think of R as a statistics system, but it is more of an environement within which statistics can be implemented. Capability of R can be easily extended by packages. There are many user-contributed packages which convers a wide range of modern statistics. These extra packages can be downloaded from The Comprehensive R Archive Network (cran).
R runs on Linux, Mac, and Windows.