What is a HDD and what should I buy ?

HDD refers to hard disk drive. The hard disk provides permanent storage of your programs and data when your computer is switched off. Hard hard disks have a capacity much larger than your computers system RAM.

Hard disk drives are characterized by

this is the amount of data that the disk can store, its typically measured in billions of bytes and labelled as giga bytes or GB.
rotation speed
mechanical drives work by store data on a spinning platter. The rotation speed in rpm refers to the speed of this platter.
interface type
the method of connected to the HDD to the motherboard.
form factor
the physical size of the HDD.
buffer size
A small internal solid-state memory cache.

The more detailed performance characteristics

disk to buffer transfer rate
The speed at which date can be read from the mechanical part of the disk to its internal memory (buffer) .
buffer to motherboard transfer rate
The speed at which data can be read from the HDD's buffer to the motherboard of the computer.
seek time
How fast a particular block of data can be located on the hardrive platter.
access time
Roughly speaking, the average seek time.

What should I buy ?

A rough rule of thumb are: the bigger the better within reason, An MS Windows system will probably require more than a Mac or Linux system. (For MS Windows just to install its system files, those in the Windows directory, takes around 20 GB). For systems running Windows 7 80GB is about the minimum practical sized hard drive capacity. A better recommendation for general computing would be 250 GB this gives a reasonable scope for general file storage. Its worth remembering that a feature movie takes around 4GB at normal DVD resolution. Systems with more than 1TB (1000 GB) are not uncommon these days but unless you have good reason more than 2 TB is likely to be wasteful and you would probably be better of spending money in other areas.

Rotation speed
The two most common speeds are 5400 rpm and 7200rpm. As a rough rule of thumb faster is better in desk top systems (where power consumption is not as bigger factor) . So, for desktop systems go for 7200 rpm for best performance. In laptops and notebooks 5400 rpm is a reasonable choice for good battery life but if you require fast disk performance 7200 rpm would be the best choice.

Buffer size
64MB is a typical buffer size and this is generally more than adequate.
A reasonable minimum would be 8MB but beyond 32MB there is little performance gain.

Transfer rates
faster is better

Seek and access times
The smaller the number the better the performance. Typically the faster the rotation speed the lower the seek times.

Serial interface (SATA) is a more modern arrangement, best to go for this.