Buying a computer 

Processor (CPU)
This is the brains of the computer- it does all the calculating. The number of a calculations it carries out (processes) is measured in gigahertz (GHz).The higher the processing  speed, the faster the computer but the more power it uses. This is an important consideration when buying a portable device.
Memory 
This is usually refers RAM (Random Access Memory), which is the temporary ,memory that the computer uses. RAM is used to run the programs that are open on the computer.The higher the RAM, the more programs you  can have open at any one time and the faster thei will respond. If the power fails, the temporary memory is lost - that's why you lose unsaved work when the coputer is switched off. You need to consider what you will be using your computer for, as graphics and videos require more RAM.
Hard Drive Size
The hard drive is the primary storage area of the computer. It is where the software, files and data are kept and where they are retrieved from. The size of the hard driv is measured in bytes - usually in gigabytes. The bigger the hard drive, the more data can be stored.
Wireless enabled
This allows the device to pick up WiFi signals, enabling you to connect to a network, connect devices together and access the Internet without wires. Data exchanged wirelessly is slower than data travelling a though cable. This is beacause data can be distorted as it travels through the air by other signals and interference travelling though the same air space. This can cause unwanted glitches and freezes in programs.
Sometimes signals may not be able to travel through walls, or certain locations may be out of range. This is an important consideration when using WiFi - the alternative is to use a cabled connection.
USB Connection
USB ( Universal Serial Bus) is the most common method of connecting devices to a computer.
Because many perifherals use this type of connection, it is good if the computer has many USB ports.
Sound Card/Graphics Card
These are devices that fix to the computer's main circuit (the motherboard), allowing computers to play and record sound and display graphics. Different cards will enhance the sound and graphics, so if you use a lot of graphics, videos, music or games, this is an important factor to consider.
Card/Graphics Card Optical Drive
The optical drive uses lasers to read data encoded on CDs and DVDs. This allows you to install software, listen to music and watch films with ease. Some optical drives also allow CDs/DVDS to be written (burned)-
this is useful if you want to transfer information from your device.

source: Edexcel GCSE Book ICT Students