"'What's the difference between an Intel Core i3, i5 and i7?' is a pretty confusing and intimidating question for the majority of people. On the surface, processors are a little tougher to get your head around than, say, a hard drive, as they are not measured by a metric as simple as capacity in gigabytes or terabytes. Here we'll explain the differences so you can choose which is best for your PC or laptop. See also: Skylake release date and models
Intel Core i3, Core i5, and Core i7 CPUs have been around since 2011. Roughly each year Intel launches new chips based on newer technology, but keeps the same names which makes things even more confusing.
What the difference between Intel Core i3, Core i5, Core i7: in a nutshell
If you want just the top-level details, here they are: Core i7s are better than Core i5s, which better than Core i3s. The main reason is performance. In general, Core i3 chips are found in cheaper or budget PCs and laptops, while the mainstream (or mid-range) systems tend to have i5. Core i7 CPUs are typically reserved for - and found in - high-end, expensive laptops and PCs.
Right now, Intel is releasign the sixth-generation Core processors - known as Skylake - and in general it's best to have a newer-generation processor. And within that generation, the best you can afford, ideally a Core i7.
There's much more to it, of course, so for those who want actual detail, read on.
What the difference between Intel Core i3, Core i5, Core i7: in detail
The Core name itself is a bit misleading. All CPUs have one or more cores, with each core being a processor itself. Most commonly an Intel Core processor will have two physical cores (dual-core) and also two virtual cores (which Intel calls Hyper-Threading).
Some, though, have four physical cores: quad-core. If you buy a Core i7 Extreme Edition, you will find up to 12 phyiscal cores. Physical cores are better than virtual cores in performance terms.
Even armed with this basic knowledge there are many other factors to consider in order to be able to say one processor is better than another. For years the highlight was always clock speed, measured in gigahertz (GHz). These days, with multiple cores and other tricks like Hyper-Threading, a processor with a lower clock speed can outperform another with fewer cores running at a faster speed. However, the operating system and software needs to support and optimally use the available hardware, and not all programs, apps and games are coded to take full advantage of more than one or two cores.
As we've said, the current generation is Skylake. The previous generation - Broadwell - never really existed as processors you could buy for a desktop PC (see Skylake vs Broadwell for more). Before that was Haswell, Ivy Bridge and Sandy Bridge. The first-generation was codenamed Nehalem.
Number of cores: Core i3 vs i5 vs i7
Sticking with Skylake, the Core i3 range is entirely dual-core. The i5 and i7 range are all quad-core, but Intel hasn't yet released and Core i7 Extreme Editions in the Skylake family.
You can find and compare the detailed specifications of all of Intel's chips at ark.intel.com.
What the difference between Core i3, i5, i7: Turbo Boost
Turbo Boost has nothing to do with fans or forced induction, but is Intel's marketing name for the technology that allows a processor to increase its core clock speed whenever the need arises. The maximum amount that Turbo Boost can raise clock speed depends on the number of active cores, the estimated current consumption, the estimated power consumption, and the processor temperature.
Core i3 processors don't have Turbo Boost, but i5 and i7s do. It means that Core i3 chips tend to have quite high 'base' clock speeds, but if you're ever concerned about power consumption, the specification to look for is TDP: thermal design power, which is measured in watts.
The Core i7-6700, for example, has a base clock of 3.4GHz, but can 'boost' up to 4GHz.
If a processor model ends with a K, it means it is unlocked and can be 'overclocked'. This means you can force the CPU to run at a higher speed than its base speed all the time for better performance.
What the difference between Core i3, i5, i7: Cache size
A processor's performance isn't determined by clock speed alone, though. Other factors such as cache memory size also play a part. When a CPU finds it is using the same data over and over, it stores that data in its cache. Cache is even faster than RAM, because it's part of the processor itself.
Here, bigger is better. Core i3 (Skylake) chips have 3- or 4MB, while i5s have 6MB and the Core i7s have 8MB.
What the difference between Core i3, i5, i7: Hyper-Threading
A thread in computing terms is a sequence of programmed instructions that the CPU has to process. If a CPU has one core, it can process only one thread at once, so can only do one thing at once (as before, it's actually more complex than this, but the aim here is to keep it simple and understandable).
Hence, a dual-core CPU can process two threads at once, a quad-core four threads at once. That's twice or four times the work in the same amount of time.
Hyper-Threading is a clever way to let a single core handle multiple threads.
A Core i3 with Hyper-Threading can process two threads per core which means a total of four threads can run simultaneously. The current Core i5 range doesn't have Hyper-Threading so can also only process four cores. i7 processors do have it, so can process eight threads at once. Combine that with 8MB of cache and Turbo Boost Technology, and you can see why it's good to choose a Core i7 over an i5 or i3.
What the difference between Core i3, i5, i7: graphics
More recently, Intel has started putting the GPU - the graphics chip - inside the CPU rather than making it a separate chip that motherboard manufacturers might include on their boards.
The latest Skylake processors all have built-in HD Graphics 530, and all have a base frequency of 350MHz. But depending on the CPU model, the highest speed (max. dynamic frequency) could be from 950MHz right up to 1.15GHz.
Gamers will still want to have a separate graphics card (see our flagship graphics card roundup) as even these new GPUs aren't nearly as powerful as an nVidia GeForce GTX 980 or AMD Radeon R9 390X.
What the difference between Core i3, i5, i7: laptop or desktop PC
It's crucial to understand that there are differences between the mobile and desktop Core i3, i5 and i7 processors. Mobile processors have to balance power efficiency with performance - a constraint that doesn't really apply to desktop chips.
However, when you're choosing a laptop the same factors apply as for desktop chips: number of cores, threads, cache size and clock frequency. Just make sure you check out the specs before assuming that a particular processor is right for you.