Depends on many things... mainly what is the application for the processor. You want to pair up a processor with whatever it will be used for. You dont want to buy a processor that just meets what your current program or application needs are unless it will only be used for that and that application or program would never become greater complex to execute. For most applications it means buying a processor that is more than you need, but within reason. That is most people dont need a Core i9 CPU and a Core i3 or i5 would be plenty. However if your a extreme gamer then you will want a i7 or i9 CPU because you will be playing games on very high and hardware demanding settings.
CPU's have Cores and the Cores are specific and named different project names. The cores of the same project name are the same generally with exceptions being different Cache Configurations.
CPU's have L1, L2 cache and in some processors L3 and L4 cache. The Cache helps the processor in that its able to store small pieces of information that the processors core(s) will use redundantly. Cache doesnt help with non redundant operations. Cache acts as a enhancement to RAM. Cache gets information to the CPU's core faster than RAM can. L1 cache is the fastest and smallest in size, then there is L2 cache slightly slower in function which can be 128k up to around 4MB or so in size ( most are 512k to 4MB in size ), then there is L3 cache which is slower than L2 and L1 but larger than L1 and L2 and faster than getting/fetching the information from RAM. Then in mostly servers etc L4 cache which is slower than the 3 prior but still faster than the system RAM fetch time. Cache also comes in Shared among cores or dedicated per core configurations.
CPU Clock Speed and Core Count then comes into play. The more cores the more multitasking and more multithreading can happen. But the program or application that is running needs to support multithreaded execution in order to make full use of all cores, otherwise it will use 1 core of however many cores the processor has. The clock speed is the Mhz or Ghz speed of the CPU such as 2.0Ghz or 4.0Ghz. A 4.0Ghz CPU "could" get 2x as many operations completed in the same time as the 2.0Ghz CPU. "Could" is stated because while a CPU waits for information there can be idle/wasted cycles of execution so it sometimes is not exactly 2x as fast in execution. Someone who is a casual computer user and a lightweight gamer would probably be fine on a 2.0Ghz CPU, however most games these days call for a Quadcore which is 4 cores and 2.5Ghz or faster clock. A gamer would want the 4.0Ghz because the processor can handle a heavier work load in the same period of time and so the game wouldnt lag at 4.0Ghz yet on a 2.0Ghz of same CPU design the processor at 2.0Ghz might not be enough to keep up with the workload to run the game in correct timing.
Lastly there are power efficient "Green" processors and normal processors and then Extreme Processors. The green processors are energy efficient and produce less heat, but this also means that they either wouldnt be able to play games or if they do play games, performance may be sluggish. These are processors that are lesser than 45 watts TDP. Normal Processors for lack of a better title for them are in the 65 watt to 95 watt TDP ( Thermal Design Power ) and these play games generally ok depending on what game and what processor core design that is utilized with multiple cores. Then you have extreme processors which generally are 100 watts or greater TDP, most are in the 125 to 260 watt TDP range. These are power hungry and in some situations the systems act like space heaters and can heat a room while they are being used such as my AMD FX-8350 8-Core 4.0Ghz with a GTX 780 Ti video card pumps out heat out of the case enough to warm my office when its used and running heavy hardware requiring games like Witcher 3. These processors the care to same money on your electric bill isnt really considered as for you want maximum performance or maximum affordable performance and so they generally run fast and hot.
Other features of processors to be aware of are instruction sets. Most processors share same instruction set support, but some have features that others dont. Instruction sets act like cheat sheets for calculations for a CPU mostly and make them more efficient as well as able to handle certain operations that otherwise might not be able to be handled. These are like SSE, SSE2, SSE3, and SSE4 instructions and a long list of others. Different CPUs have and lack these. Some share same instruction sets and others have their own unique ones. *Most people dont have to worry about these unless they need a processor for specific calculation or multimedia needs. Most consumer processors have a good mix of most of what you need.
https://en.wikipedia.org/wiki/List_of_instruction_setsLastly a Mobile CPU to a Laptop is "drastically different" than a Desktop CPU for a Desktop Computer. Some people see Core i7 and think cool I have a Core i7... but benchmarks of a Laptop Core i7 and a Desktop Core i7 will show that the Desktop Core i7 of the same clock will perform way better than the laptops CPU. This is because Intel for example is using the i7 name in marketing and people not knowing any better may thing the Laptop and Desktop are the same CPU but the laptop is nice and compact so I want to save space so I will go with the laptop, but what they just did is settle for lesser performance usually without knowing it. Them not knowing it is generally because even the mobile Core i7 is more than plenty than more would need, but years down the road all of a sudden that laptop seems laggy say 5 years in age when software gains execution complexity and so the person who bought the desktop might not feel lagged but the laptop user is starting to feel the lag of their weaker in processing power Core i7. *The TDP rating of the CPU is usually a good indicator of performance in newer processors. If your looking at a processor that is a TDP of 4.5 watts then its an electron sipper and it will likely lag out pretty easily on stuff when it runs into heavy hardware needy software execution, but the laptop that is 45-60 watt TDP will run way better. So dont be tricked into marketing names and look at benchmarks before making your purchase as well as look at others reviews. I avoided purchase of a CPU years ago because it was found out that the CPU's hype in how good it was suppose to be was all marketing lingo and in benchmarks it performed unexpectedly poorly. In a newer design they cleaned up the inefficient design and made for a better performing processor.
I personally never buy anything right when it first comes out and let others be the guinea pigs. Most products released perform as intended. Most arent flops, but I also dont like paying crazy amounts of money for a computer, so I generally buy a good processor that is a year old or so. And this way instead of paying a large price tag the prices have then come down from their release date pricing and the most I have ever paid for a CPU alone was $159.99 for my AMD FX-8350 4.0Ghz 8-core 125 watt TDP. I built another 8-core system with a same core design AMD FX-8300 3.3Ghz 95 Watt TDP and I actually prefer this system more than its 700Mhz faster and 30 watt hotter running brother. Cooling fans on both of these systems run drastically different. The same heatsink on both, both heatsinks came from AMD as stock coolers and the 3.3Ghz runs cooler at around 48C gaming and fan runs at a slower speed to cool the CPU, however the 4.0Ghz system the processor runs hot around 62C gaming and becomes a space heater in my office as its pumping out excess heat as its driven heavy at 4Ghz. *Note: I have Cool n'Quiet enabled in my BIOS so the CPU's will underclock when idle to 1.2Ghz for example vs running full tilt to 4.0Ghz.
If your looking to build or buy a system, share with us your computing needs ( software titles and game titles and application needs such as multimedia etc ) and we can assist you in making the best decisions on what would be a good match for you to save you some money maybe, but most importantly helping you avoid buying a sluggish lemon of a system. * Lemon systems exist. These are systems that are advertised with specs that cause impulse shoppers to buy into a system based on specs, and when they get these bargain systems they find out shortly after the warranty expires that the main board for example or power supply was a junk quality component and it dies on them and they are out the money spent, or forced to spend more money to correct for quality deficient parts that lead to deficient operation.
