Everything occurs at the CPU, essentially. And it is sequential; one at a time. NOTHING on your computer runs simultaneously, even on dual processing because one of those CPU's has to wait for the other to finish :P. It's simply breaking up what one CPU would have done anyway, ONLY IF the programmer designed it for duo core (threading according to that architecture). Oftentimes you can hit ctrl + alt + del and see a process like a game consuming 50% of your CPU because that game, like most every other program to date isn't designed for duo core. ANYWAY, back on subject:
Everything in the computer occurs in steps of finite time, ONE by ONE. This time is known as the system clock, which runs at a certain MHz. Let's say its 133Mhz. However the CPU runs faster, yet on the same clock speed. How? It runs, as set in the BIOS (check yourself), at a multiplication factor of the system clock. So say it runs at 9x (system clock), or 9 x 133Mhz = 1297 or 1.3 GHz. So the CPU can do 9 operations before System bus (which runs at the speed of the system clock) will be accessed (if needed) to get something from RAM, an HDD, or a device. As a computer user, the only thing you ever do on a computer is play around with the CPU, using an application to do this for you. THE CPU then reads/writes to every thing else in the computer... the CPU controls the rest of the computer. As a programmer you control the CPU much more closer. Obviously you can't do shit on a computer if you don't understand it, and you can see where programming comes in as a need to know. Also, multiple programs ONLY seem to run on a computer simultaneously, but they are, in reality, being given a small fraction of time to run, in a priority queue, then kicked off the CPU by he OS's CPU scheduler, given to the next process in line. For the noob, process = program. Program = simple user level talk.
The goal of any hack is to get access to the CPU essentially. Obviously root or and admin account would be prime access to run the best applications BUT if you can inject your own code in there during a user session (often called shell code) to give you such an account or higher level system privilege then you are in.