A PC can't become one without a CPU and a Gaming PC will have alot of trouble with processing the games information if the CPU is just from your standard home PC.
Knowing what you require
Do you want to edit videos/audio/photos with this PC or just game? But are you going to want to build a budget $500 PC or a $3000?
The CPU needs to handle what the rest of PC is capable of and what you want to get out of it.
CPU - Central Procession Unit, refers to the whole piece itself and if it is a multi-core processor than it can also be used when talking about each core itself.
Socket[s] - Mechanical component(s) that provides mechanical and electrical connections between a microprocessor and a printed circuit board (PCB). This allows the CPU to be replaced without soldering.
GHz, Clock Speed - The digital chips on a motherboard are keep in sync with each other by the clock signal (a stream of pulses) of the motherboard. You can think of it like a “heartbeat” of the computer. The faster the clock pulses, the faster the computer runs; but, the clock can’t run faster than the speed rating of the chips, or they will “glitch” and drop data. As chip technology has improved, the speed that chips can run at has gotten faster. The CPU runs faster than the rest of the motherboard (which is clocked at a fraction of the rate of the CPU). Clock speed is measured in units of cycles per second, which is called a Hertz (Hz). Computer boards and CPUs run at rates of millions and billions of Hertz, megahertz (MHz) and gigahertz (GHz), respectively.
Cache - Where small chunks of info are stored, while the system is operating, so that the CPU can access information faster than going to the RAM and getting it from there or in some extreme case, accessing the info on a HDD shudders .
Printed Circuit Board [PCB] - Mechanically supports and electrically connects electronic components using conductive tracks, pads and other features etched from copper sheets laminated onto a non-conductive substrate. PCBs can be single sided (one copper layer), double sided (two copper layers) or multi-layer.
Cores - Refers to the number of CPUs on the same PCB or integrated circuit.
Unlocked [CPU] - When the multiplier of a CPU is allowed to be changed, this is the main way a user can overclock a CPU, but not the only way. By far the easiest and causes less errors elsewhere on the system
Locked [CPU] - When the multiplier of a CPU is not allowed to be changed.
Overclocking - Forcing a CPU to run at a higher clock speed than listed on the manufacturers specification. Overclocking a CPU increases its performance, although it can result in system instability and may cause damage to the CPU.
TDP - Thermal Design Power, is the maximum amount of heat generated by the CPU that the cooling system in a computer is required to dissipate in typical operation. This is without overclocking CPUs as taht will inherently increase the TDP dramatically.
Data Bus - The data bus is the multi-lane electrical highway of connections that link the CPU to the other chips on the motherboard, such as the RAM memory and I/O controllers. It is also called the front side bus (FSB). The word size of the data bus determines how many bits can be moved simultaneously along it.
Instructions Per Clock [IPC] - "The number of instructions per second and floating point operations per second for a processor can be derived by multiplying the instructions per cycle and the clock speed (measured in cycles per second or Hertz) of the processor in question. The number of instructions per second is an approximate indicator of the likely performance of the processor." - Source
If you have any more definitions you think that need to be added or fixed even, message me, /u/Bosses_Boss, and I'll have it added.
Things to avoid
Assuming is the name of the game here.
Upgrading on a cycle.
Thinking you'll be needing a certain specific CPU for what you want to do, in most cases, new builders are surprised by how little the CPU matters, but this does depends on the needs and wants of your specific builds purpose.
Using a percentage of the total build cost to find the CPU for that build.
Things to seek
Facts are the name of the game here.