I basically tried R for some time. I found out that the syntax of R is pretty much similar to S-Plus. what's the relationship between this 2 software. I know R is open source and S-Plus is a commercial software.

and could you please give a comparison of R and S-Plus?

Seems both R and S-plus are dialects of the same language, S.
S-plus is licensed by the Bell Labs where S was developed to a private company which adds GUI and such to prettify the interface.
R is open source and is very actively developed. I believe the original creator of S is very involved with R. It also appears the language to do cutting edge
statistical analysis in academia and also enterprise, such as financial engineering.

I would agree with Igor. S-Plus and R had their roots in S. S-Plus is proprietary and was acquired a while ago by TIBCO and renamed TIBCO Spotfire S+. I have to believe that R is receiving more active development given the enormous academic user base world-wide.

Given their their roots, the two languages are very similar, but be careful. They can differ in their scoping rules. In particular, they differ in how free variables (variables within functions that are not assigned values within the function call, and are not assigned values locally) are ultimately assigned values.

To learn more, search on the phrase "lexical scoping in R".

subject: what's the relationship between R and S-Plus.