google.com is a good place to start. we don't mind answering questions but when it seems you have made no personal effort to find the answer it is a bit annoying as there would be at least several hundred thousand pages on this topic.
quick summary: memory frequency is linked to the frequency of your processors FSB. so in the case os SDRAM and anything before it runs at the same speed as your FSB. DDR ram runs at double the frequency of your FSB. by overclocking your processor your memory will be overclocked unless you use a "memory divider".
a memory divider is a ratio or a percentage that can be used to change the frequency the memory runs at relative to the processors FSB. in other words you can use it so the memory runs at a chosen fraction of the FSB e.g. 9:10, the memory will run at 9 tenths of the FSB so at stock speed of 200mhz that would have your memory running at 180mhz. as you start to overclock the memory will remain at that fraction of the FSB so if you hit say 250mhz your memory would be running at 225mhz (DDR450)