Generally, when we speak of a "computer", we are referring to what is more formally known as a "stored program digital computer". This means that the program that controls the processing of data is not something completely different or distinct from the data itself.. In fact, the program is data, and for the most part, it is manipulated by the machine in the exactly same way as the "ordinary" data. And because they are essentially the same, the program and data are stored together inside the machine, hence the term "stored program". The "memory" of the computer is where all this data is stored.

The difference is simply that things are normally arranged so that the computer uses the "program data" in its memory as instructions on how to process the "ordinary data", also in its memory. This similarity of programs and data enables some programs to process other programs as data, which in a limited way, enables the computer to "program itself", and this is what gives the stored program computer its great power and flexibility.

Of course, we also need some means of putting the raw data and the program into the memory, and a way to get the processed data out afterwards. That is the job of  the "Input/Output units" (I/O devices). Also, the computer needs something to read the instructions of the program, and to actually process the data. This is done by the "Central Processing Unit" (CPU).

These three things, the CPU, Memory and I/O devices, are the classic components always described in any explanation of a stored program computer. This page is only concerned with the memory component of computers, and how memory technology has progressed. 

We must also distinguish "memory" from "mass storage", which means things such as disks and tape. Though these also store data, they are considered to be I/O devices. They hold large amounts of data, but the computer cannot use this data until it is first brought into the memory. The CPU can instruct the I/O devices to read and write data to and from the memory, but it can only fetch instructions from programs that are already in memory, and can only process data that is also in the memory. 

To understand more about what data is and how it relates to computers see "Binary Digital Data"