AVR, ISP?

What is AVR programmer? How is it different than ISP? I noticed that the AT89Sxx series has some pins meant for ISP... arent Rx and Tx sufficient?

Most of the AVRs are programmed using ISP only..you will find rare parallel programmer for AVRs.
in case of AT89S series.. yes they can be programmed via Tx and Rx.. you can make use of PonyProg software for programming AT89SXX microcontrollers..

so what is difference does it make whether i program with ISP programmer or directly using Rx Tx lines. Is there any advantage of using the extra hardware for programming?

well.. programming using Rx tx is also the same ISP because you don't need any extra hardware for that got it?
I think you have a different or wrong image of ISP in your mind..
ISP - In System Programming
It means you need not to take your microcontroller out of the circuit if you want to program it.

in 8051s mostly they use rx tx pins to program where as in case of AVRs MISO and MOSI or say SPI is used to program the ICs.
Both have ISP interface but just the ways of programming is different.

I am still not clear. Suppose i have AT89Sxx. It has MISO, MOSI pins as well as Rx tx. can both be used to program? And i believe MISO, MOSI will require some h/w in beetween.

you need to check the datasheet then.. I give you an example.. like AT89C51RD2 (8051 variant) has both MISO MOSI as well as RX Tx but the ISP works on RS232 i.e. Rx and Tx. This information can be obtained from the datasheet of your controller.
Tell me which AT89S you have?

I am thinking of building this board. I have option to use AT89S51, AT89S52, AT89S53, AT89S8252.

is it a tested one?
you can also try visiting www.lancos.com
Pony Prog supports AT89S53 and AT89S8252.

probably working. PCB artwork is also provided. pony prog schematic seems to be relatively complicated.

as per you feel better make it and tell me if it worked or not.. i will add this circuit to my inventory