DDR laptop memory, otherwise known as DDR1 (though this isn't an official name instead referred to as just DDR) or DDR SDRAM is the oldest and slowest. It's predominantly available in capacities up to laptop i7 1GB (per module) and at speeds of up to 400MHz (effective).

DDR2 is an evolution to DDR with some internal enhancements to boost performance and usually available in capacities up to 2GB (per module) and speeds up to 1066MHz (effective).

DDR3 is presently the latest form of laptop DDR memory and is yet a further evolution of DDR2 boosting speed a further notch. It can be found in capacities up to 4GB (per module) and speeds up to 1600MHz (effective).

Notice that we could have said DDR2 SDRAM just as DDR3 SDRAM. For simplicity sake, stick to remembering that there's DDR, DDR2 and DDR3 because quite frankly this is what you'll encounter most! Luckily, the industry seems to prefer the shorthand naming convention.