Lets start it this way:
i dont know if you already know this but this is the only way i can help,
NO 1.! cpu: amd and intel, thats the two brand your after if your going to build computer, the latest sockets are am3(amd) and 1156 and the other one 1336(intel) the amd cpu for this are the athlon(dual cores) II phenomII (dual, triple and quad cores ) and sempron II (single cores) in the intel there are core i7((socket 1336)this are quad core with each cores have 2 threads and it means more stable speed for more applications open(some says its like a 8 core cpu with 4 virtual and 4 physical)) the other one is the core i5((socket 1156)this is the cpu before the corei7 its less powerful but also less expensive)
and there are rumors that the socket 1156 will become the standard and not the 1336 even its faster, remember its rumor.
NO 2.! Motherboard,.. here is where you really see what are the sockets are for, if your building a system,.. the mother board is the one that will determine what cpu are you going to buy, what graphics card are you going to plug in(all cards will do as long as its same slot(pci-e(new) or agp(old)), what i mean here is if your going to single card solution or multiple card) and type of memory (ddr,ddr2,ddr3) and so on,.. those 3 are the vital(cpu and graphics card, memory)things the the motherboard will tell,.
so choosing your motherboard is one of the most important thing be sure to get the right one for your needs.
NO 3.! The Memory there are 3 types ddr(its the same from what other called ddr1),ddr2 and ddr3, there are more older types thats not needed here,. things you need to know when choosing these 3,.. amount of memory, frequency, and latency,. amount of memory is ofcourse if its 2 gigs, 4 8 or 16 or more, frequency normally ddr starts from 133 i think to 333 and ddr2 starts from 333(that is rare this days) to 1200(1066 is the common this days) and the ddr3 from 1200 up to 2000mhz or more, frequency are accompanied with MHz/GHz, the latency, it is the timing of your memory, the lower the better,, you commonly see this as cl5-5-13-19 something like that written in the memory module(in the memory itself)
note: the higher the frequency(that is good) the higher the latency(that is bad) and the lower the frequency(that is bad) the lower the latency(that is good),.. and if you can purchase from good companies they have memories that are high frequency with a low latency(generics are bad).memories also have pins, normally memories nowadays are 240 pins and if your motherboard is a 240pin dimm(dual inline memory module, thats the slot where you put the memory) then you have to buy a 240 pin rams
remember that because ddr3 are high frequency most ddr3 ram are also high latency,. but as days go by they are already make improvements,.
NO. 4! graphics card two major players nvdia and ati/amd(amd and ati are now one)what you are looking for when buying graphics card? clock speed,memory clock and shader clock, memory clock this is like the frequency of your normal memory the higher the better(this is the speed between the gpu (the cpu of our graphics card)and the memory thats why high is good), clock core, this is the base clock of your graphics card the higher the better(the cpu of your graphics card that do the process thats why high is good ), shader clock is also the higher the better(this is the one who do the work in you screen but not only this there are more), stream processors this are the things that make what effects you see in the monitor, the more of these is the better because if there are more effects and less stream processors you frames will drop(fps frames per second where in if you drop less than 28 it will give you some lags and slow down the game)
NO 5.! storage, two common types hardrive and solid state drive, hard drive is the common nowadays, it have some magnetic disk where the data is stored and the more rpms(revolution per minute) the fast response, normally 7200 rpms are the common rpm, and there are also 1000 rpm, the solid state drive is chip based storage, there is no disk to rotate so less vibration and noise its better but also more expensive in only few company have big storage amount for the ssd(solid stat drive) you will commonly find 64gig 128gig and 256 gig,.. maybe some company have more than that(but to much expensive), ther e are 2 type of ports sata(there are sata I 1.5 mb/s and sata II 3.0mb/s) and pata(the old one) just buy the sata1 or 2 because its better,..
NO. 6! power supply avoid buying generic psu(power supply unit), generic psu have lower amperage its better to buy 500w quality psu than a 650 generic psu,.. quality psu are more stable more life span and less danger to your other components,.. believe me you dont want to short out your motherboard because of bad psu, if your motherboard have 24 pin and 8pin for power, buy a psu that also have 24 and 4 and depends on your graphics card if you need some pci-e power plugs but most of quality psu have all you need these days,..
NO. 7! Case, what you are looking for this are ventilation and cable management,.. it should have atleast 1 120mm intake fan and 2 120mm exhaust fan and some cable management holes to hide the cables that give better airflow,.. and also remember that long graphics card should fit on it,..
case sizes:
Full tower case
Mid tower case
HTPC case(home theater pc case)
i cant remember the others but the first two are what you commonly see.
designs are about your own preference.
NO. 8! optical drives are about to you just be sure that it can hold dual layer dvds and bluray if you want to.
these are the basic thing you need to run your gaming system,.. its depends on you what you want to choose,..
i hope you appreciate it,... and just tell me if theres something you dont understand or any question..