difference between TFS and RTC

I don't agree with this "quick and simple" answer. The RTC Client for Visual Studio is a natively implemented rich client with functionality that is comparable to the RTC Eclipse Client - it's not merely a plugin to fill in gaps. It currently supports most of the developer use cases supported in RTC Eclipse, and with each release it'll strive to get closer.
You'd be the best judge of whether it suits your needs or not - and for that you can easily download the 2.0 version for RTC and play with the client for Visual Studio.

one of the client is currently using Microsoft Visual Team Foundation server. does he still require RTC. what is the difference between TSF and RTC?

I will give the quickest and simplest answer possible. If you are .NET shop then TFS is your best bet, and JAVA shop will go with RTC. Even tough both has plugin to the other side, it merely just fill in gaps.

Rational Team Concert and Microsoft Team Foundation Server are both trying to solve similar problems, but in different ways. They both support an integrated team environment with build, scm, work items at the center of the platform.

I won't go into a feature/feature comparison, but there are differences. I suggest you read-up on both jazz.net and microsoft sites for more details and to make up your own mind ;)

- Team Concert supports many different heterogeneous client and server environments and operating systems (eg, oracle, sql server, db2, visual studio, eclipse, etc...).

- Team Concert's support for Agile planning is top of the line.

- The Jazz family of products allows more lifecycle integration with requirements (Rational Requirements Composer) and tests (Rational Quality Manager), for which no coverage is available on the MS TFS server.

Cheers,
Jean-Michel

As a user that is caught between the two, I agree wholeheartedly with Seerupa.  The two tools are well suited to handle agile development.  By far RTC is the richer tool.  Along with that rich environment comes a lot of complexity and cost.  If your client is in TFS I would stay there.