Hello ATR1153;
Some of the bigger 17" laptops actually have two hard drive bays that allow 2 drives to by physically installed inside the laptop.
For most laptops you'll be using a 2nd hard drive as a removable, external drive, usually plugged in by the USB port.
If you already have the hard disk you can buy an inexpensive case to hold the drive with the proper connections.
Just be sure to use the right type of case for your HDD.