Primary memory and secondary memory are structured differently. Primary memory is on microchips called RAM or random access memory. Each chip has a huge grid of tiny switches. Each switch stores a single bit of memory, which the processor can access at any point by sending an electronic signal to the memory. Secondary memory is generally stored on a spinning magnetic disk. Memory is stored in circular tracks similar to the grooves on a record and a small read/write head floats above the surface of the disk like the tone arm on a record player. When the computer wants to store or read information on the disk, an electric motor moves the arm to a particular point over the disk and the read/write head either writes to the disk by making a series of magnetic pulses or reads from it by measuring the magnetic fluctuations in the surface of the disk. Some modern computers use what is called a "solid state" hard drive. The solid state drive stores information in a grid and has no moving parts, making it closer in structure to RAM than it is to a hard disk.
Because primary memory requires no moving parts, it is almost instantaneous. Hard drives, by contrast, are much slower, since the computer must swing the read-write head across the disk every time it wants to access a chunk of data. Because of this, computers run programs in primary memory. If the computer runs out of space in the primary memory, it will create a swap file on the hard disk, a file that functions as backup primary memory. The computer will only use this file when there isn't enough primary memory, since using the swap file slows things down.
Secondary memory is slower than primary memory, but it does have two advantages: size and persistence. Secondary memory is much cheaper per unit than primary, and many hard drives now have a terabyte (1,000 gigabytes) or more of memory. By contrast, computers have at most a few gigabytes of primary memory. Even more importantly, secondary memory has persistence. Primary memory is a grid of electronic switches, and the information is only stored as long as electricity is flowing through those switches. When the computer is turned off, all the information in primary memory is lost. Secondary memory, by contrast, is stored as magnetic fields either on a disk or in a grid. Even when the computer is turned off, the memory remains. Because of this, files and programs are stored into secondary memory, then loaded into primary memory when the computer needs to run or access them.