There are two numbers that are important - rotational speed and access latency. Rotational speed is how fast the pplatters rotate - the faster the better. Typical speeds are 5400 rpm (revolutions per minute) and 7200 rpm. There are faster more costly drives. Typically the 5400 rpm are 2.5" form factor disks used in laptops while the 7200 rpm are 3.5" form factor disks used in desktops - where the higher power draw doesn't matter. The higher the better.
The latency speed is how fast, on average, the disk read/write heads can seek to a sector(data unit) on the disk. Typically thee are in the 10 ms or sub 10ms range. The lower the better.
Some drive manufacturers will quote sustained data transfer speed in Megabytes/s - the higher the better.