SDRAM modules usual have 168 pins (DIMM slot) for desktop use (can be 144 or 100 pins for laptops and printers), as opposed to 72 pins (SIMM slot) for most EDO modules. Some older motherboards support both EDO and SDRAM memory and have both sets of slots.
However you must remove the EDO modules to use SDRAM. Typically, if you fill both sets of slots, the motherboard will recognise and use either the EDO or the SDRAM modules, but not both.