Memory or Random Access Memory (RAM) allows your computer to multitask. It temporarily stores data and the higher number of GB (Gigabytes) means more applications can be running in tandem without slowdowns. Some intensive applications for gaming or video editing, require more RAM for optimal performance.
Memory described as 'DDR' stands for 'Double Data Rate' and processes data approximately twice as fast as a standard memory module. When a number appears after DDR (ie DDR3) that type of memory is approximately twice as fast as the previous generation.

The speed and power of a processor (aka the 'brain' of your computer) determines how well and how fast your computer can do things like create spreadsheets, play games, browse the internet, run software, and more. The more cores a processor has, the better it will be at doing multiple things at once.