What is gpu? which gamesm can i play

so what are your specs

and you questioned what is gpu the answere is it is a computer component which stands for Graphics Port Unit

GPU = Graphics Processing Unit

are you buying i3 with windows7 and what do you mean that i3 built-in graphics can run halo or call of duty yes it can.you can play all newer games but in lower and medium setting because build-in graphics are not too powerfull as compared to external gpu's.
GPU:graphics processing unit one of the most important component in computer that will improve your games much more with a decent graphic card like hd 5450,hd 5770,hd 6770,gtx 460 and so on.

but you can't run new and latest games on maxed out with that thing