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In Activity Monitor, there are a couple fields: "PID", "Process Name", "User", "%CPU", "Threads", "Real Mem", and "Kind". I'm not familiar with the inner workings of computers and the related technical terms, but I've always thought the "%CPU" and "Real Mem" would be synonymous. However, when I look the running processes in Activity Monitor, I see this:

 PID | Process Name | User | % CPU | Threads | Real Mem | Kind

Evident from the image, "Real Mem" and "%CPU" are not synonymous.

What's the difference between real memory and CPU, and what are there effects on the performance of my computer?

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To answer your question about performance, it really depends on what you're wanting to do.

CPU: To put it simply, CPU speed defines how quickly your computer can process data. As you're asking the question in a Mac forum I'll assume you want to by a mac. The most basic MacBook Air laptop has 1.7GHZ dual core processor, which means it has two 1.7GHZ processors. For basic computing this is more than enough, but if you wanted to do something a bit more labour intensive, say photo editing or graphic design or high end gaming, it'd probably be a bit slow. If you look at the slightly more advanced machines apple has on offer like the macbook pros or the iMacs then the processors are more in the 2.5GHZ - 2.9GHZ range, this is plenty for most high end applications. Finally the top of the range machines are the Mac Pros. They have quad core, 8 core and 12 core options upward of 3.2GHZ. These are great machines if you have the money and will make photoshop run a little faster, but not really a necessity unless you want to do some really high end stuff like Animation or complex video editing.

Memory: Memory consists of what can be considered your long term memory (your hard drive) which is sometimes called "storage" and your short term memory or RAM (stands for Random Access Memory if you're geeky enough to be interested) which is what the term "memory" is normally referring to. Ram controls how much your computer can store in the short term to process. Most basic PC's and the most basic macbook airs come with 2 gigs of ram nowadays which is more than enough for basic applications. However, again if you're looking to do photo editing or graphic design or online gaming then you really should be looking at between 4g and 8g otherwise your computer won't have enough memory to process lots of different functions at the same time. And of course, if you're a budding Peter Jackson about to make the next lord of the rings film and you need to be able to render an army of mediaeval monstors, then the mac pro ranges from 16g to 64g of ram.

Hope this answers your question better.

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CPU is the Central Processing Unit. It's an Intel microprocessor. It performs calculations (or more precisely, computations).

Memory is temporary data storage space used by active processes being executed by the CPU.

If we were talking about cars the CPU would be the engine and the memory would be the seats (or possibly the cup holder).

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Thanks. I didn't really make this clear, but I was wondering about the effects of each on my computer's performance. For example, if a process is using up a lot of the CPU's processing capacity, will my computer lag/slow down? –  qegal Aug 2 '12 at 6:43
    
@qegal, then you should consider editing your question, because you are asking what the difference is between CPU and memory. –  Gerry Aug 2 '12 at 6:58
    
@Gerry - Okay. I updated the question to make my question clearer. –  qegal Aug 2 '12 at 6:59
    
The short answer is that if either is over-utilized or saturated you will see performance problems. –  bahamat Aug 2 '12 at 17:19
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