[Merkuri]

The computer memory is the physical devices used to store instructions and data.

Note: When I give some access or seek time this is the average case.

Bit stand from BInary digiT, it's the smallest measurement unit in the computer memory and has value 0 or 1 (off or on).
Byte(I may refer to it as memory cell) is an array of 8 bits.
Word is an array of 4 bytes.
Those units are most common, however there are systems where 1 word is 2 bytes for example.
Primary Memory

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This memory is used to store the instructions for a program and the data that the program need. This memory is volatile. She is fast,the data gets lost when power is switched off(this is why the memory is called volatile) and has small capacity. These memories are not as fast as registers. The data and instructions required to be processed earlier reside in main memory.
RAM

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RAM stand from Random Access Memory, this is volatile memory as I said above. You have random access to any memory cell. That is, any bit can be access without touching the others. There are two types of RAM. DRAM and SRAM.
Types of RAM
DRAM
DRAM stand from Dynamic Random Access Memory. This memory must be constantly re-energized(or refreshed) or it will lose the content which the memory holds, this is why this memory is called dynamic. This memory access time is around 60 nanoseconds.
SRAM
SRAM stand from Static Random Access Memory. This memory has access time 10 nanoseconds. As the name says this memory don't need to be re-energized as DRAM and it's faster. But it's more expensive to produces, this is why this memory is used mainly for cache.
SDRAM
SDRAM stand form Synchronous Dynamic Random Access Memory. This memory has much higher clock speed than conventional memory. SDRAM actually synchronizes itself with the CPU's bus.
There are other types of RAM, however I won't name them here since they aren't critical to know.
DDR Module
Before I continue, I want to say that there are other modules which aren't very actual today, however if anyone what to learn more about them he can leave a commend or send me a PM I will name them, and find a good source about them.
DDR stand from Double Data Rate. As the name says this module double the amount of data. Basicly SDRAM can send or receive data for one clock cycle. DDR can send and receive data for one clock cycle.

ROM

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ROM stand from Read-Only Memory. This memory is used to store permanently data on personal computers. It is permanent and non-volatile. Since it's read-only the data there can't be changed. There are many types of ROM like Programmable Read-Only Memory (PROM), Electrically Programmable Read-Only Memory (EPROM) and other for which I wouldn't write now. Most computers have small amount to store critical programs usually the BIOS is stored there.

Secondary Memory

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This type of memory is know as external. This memory is non-volatile. It's slower then the primary memory and it's used to store programs and information permanently. The contend of secondary memory is transfered to the main memory, after this operation, the CPU can access the data. Example for secondary memory are harddrives, cd-rom, USB memory and others.
I pretty much failed to explain about harddisk and interfaces. However I find source in Internet that do the job.

Code:

    http://www.youtube.com/watch?v=rREkzeoJz1s
    http://www.pcworld.com/article/18693/article.html?page=3

First view the video and then read the article. It's extremely hard to explain how harddrive work in short without making a video.

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Images

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RAM memory

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ROM memory

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RAM-ROM

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Please report for any mistakes

[Jolly]

To be honest the format of the Tutorial is quite un-pleasant and the first part of it is rather lacking in detail.

[Merkuri]

To be honest the format of the Tutorial is quite un-pleasant and the first part of it is rather lacking in detail.

This is short introduction not "all you need to know about memory". I don't think that someone who just start dealing with computers would want to know about stack and memory managers in OS nor the circuits.