Consult your laptop’s manual and make sure you purchase the right type of memory for your laptop:

SDRAM: New laptops no longer use SDRAM (Synchronous Dynamic Random Access Memory), but many older machines can benefit from an SDRAM upgrade. Buy the SDRAM that fits your motherboard, either 100 megahertz or 133 megahertz. You will find the exact specifications in your laptop manual.

Double Data Rate SDRAM: DDR SDRAM allows memory to deliver data to the computer processor twice as fast as SDRAM.

DDR2 SDRAM: DDR2 SDRAM has a faster clock speed than DDR SDRAM. DDR2 also has other tweaks to make it work more reliably.

Make sure you buy SODIMM memory designed for laptops, which is smaller than the full-size DIMM memory used in desktop PCs.