From the abbreviations, you can already guess that these two are types of RAM or Random Access Memory. These are types of RAM that may have been more popular in the past. Over the past years, more things have been offered to people so they would like to try things that can be considered to be more modern. These two have differences too.
SDRAM is still the prototype for the RAM that is available in laptops and computers right now. RDRAM is considered to be an alternative type of RAM for people who are not too sold on SDRAM. The main difference between the two has to be the price. The modules for RDRAM are known to be thrice the amount of the modules of SDRAM.