Main Parts of Desktop Computer.

Switched mode power supply

Power Supply/PSU power supply unit, converts outlet power, normally power available at our place is AC power and all computer components works on DC power. Switched mode power supply is used to convert AC power in to DC power, It also control the voltages and currents for the various internal components.
smps

Motherboard

Motherboard/mainboard As the name indicates, this is the main component of the computer, everything else connects to the motherboard. Processor/CPU central processing unit, the "brain" of the computer, most actual computation takes place here.

Random access memory

RAM random access memory, the "short-term memory" of a computer,RAM store any program temporarily and remove when program is closed. When you open any program, it goes in to RAM because your CPU can get it a lot faster from RAM, Data in RAM is lost when the computer is powered off, thus necessitating a hard drive.
ram

Internal Hard Disk

Hard Drive/Hard Disk the "long-term memory" of the computer, used for persistent storage i.e. the things stored on it remain even when the computer is powered down. The operating system, and all your programs and data are stored here.

Optical Drive device for reading/writing optical disks. May read CDs, DVDs, or other optical media, depending on the type. It is essential for installing many operating systems and programs. It may be able to write some of these discs, as well. Some people like to have two such drives for copying disks.

Video Graphics Card

Video Card/Graphics Card/GPU does processing relating to video output. Some motherboards have an "onboard" GPU built in so you don't need (but may add) a separate video card. Otherwise, you will need a video card. These plug into a slot on the motherboard and provide a place to connect a monitor to your computer.
Graphics Card

On top of the internal components listed above, you will also need these external components:

Computer Keyboard

Keyboard for typing on. Many motherboards won't even boot without a keyboard attached.

Computer Mouse

Mouse for pointing and clicking. Unless you chose a text-based operating system, you will likely want one of these.
Mouse

LCD/LED Screen

Monitor This is where the pretty pictures go. They come in many forms, the most common being CRT and LCD.
monitor

These are the parts that a standard PC will use. We are not considering such esoterica as headless, touchscreen, or voice-controlled systems. You might want to make a check list (perhaps using a spreadsheet) of parts to use as you go about your process of research and selection. That way you won't find yourself sitting down with a pile of brand new hardware only to find that you forgot an essential component.