Back Panel

he back panel is the portion of the motherboard that allows you to connect external devices such as your monitor, speakers, keyboard, and mouse. As can be seen in the picture below, the back panel is on the edge of the motherboard. When installing the motherboard this side of the motherboard would be on the back side of the case and are inserted into the cases I/O plate.

Processor

A central processing unit (CPU) (formerly also referred to as a central processor unit) is the hardware within a computer that carries out the instruction of a computer program by performing the basic arithmetical, logical, and input or output operations of the system. The term has been in use in the computer industry at least since the early 1960s. The form, design, and implementation of CPUs have changed over the course of their history, but their fundamental operation remains much the same.
A computer can have more than one CPU; this is called multiprocessing. All modern CPUs are microprocessor, meaning contained on a single chip. Some intergrated circuit (ICs) can contain multiple CPUs on a single chip; those ICs are called multi-core processor. An IC containing a CPU can also contain peripheral devices, and other components of a computer system; this is called a system on a chip (SoC).
Two typical components of a CPU are the Arithmatic Logic Unit (ALU), which performs arithmetic and logical operations, and the control unit (CU), which extracts instructions from memory and decodes and executes them, calling on the ALU when necessary.
Not all computational systems rely on a central processing unit. An array processor or vector processor has multiple parallel computing elements, with no one unit considered the "center". In the distributed computing model, problems are solved by a distributed interconnected set of processors.

Hard Disk

A hard disk drive (HDD is a data storage device used for storing and retrieving digital information using rapidly rotating disks (platter) coated with magnetic material. An HDD retain its data even when powered off. Data is read in a random-access manner, meaning individual blocks of data can be stored or retrieved in any order rather than sequentially. An HDD consists of one or more rigid ("hard") rapidly rotating disks (platters) with magnetic heads  arranged on a moving actuator arm to read and write data to the surfaces.
Introduced by IBM in 1956 HDDs became the dominant secondary storage device for general purpose computers by the early 1960s. Continuously improved, HDDs have maintained this position into the modern era of severs and personal computers. More than 200 companies have produced HDD units, though most current units are manufactured by Seagate, Toshiba and Western Digital. Worldwide revenues for HDD shipments are expected to reach US $33 billion in 2013, a decrease of approximately 12% from US $37.8 billion in 2012.
The primary characteristics of an HDD are its capacity and performance. Capacity is specified in unit prefixes corresponding to powers of 1000: a 1-terabyte (TB) drive has a capacity of 1,000 gigabyte (GB; where 1 gigabyte = 1 billion byte). Typically, some of an HDD's capacity is unavailable to the user because it is used by the file system and the computer operating system, and possibly inbuilt redundancy for error correction and recovery.