Hard disks are used to store data.

They are electro-mechanical devices that have platters spinning at high speed within a protective metal case.

If 'performance' is measured by how much data the system can store, then purchasing a higher capacity hard disk will provide an immediate boost.

For example replacing a 60 GByte disk with a 120 Gbyte disk, doubles the data storage space.

Another option is to install a second hard disk to run alongside the first - again capacity has increased. An additional benefit of having a second hard disk is that you are able to very rapidly back up your data from the first one. Then, if a disk fails (and eventually it will) your data is still safe.

Sometimes it is important to be able to access the data a bit more rapidly - reading off video files for example. In this case purchasing a hard disk that spins at a faster rate will provide a moderate boost to performance. The diagram below shows the platters and read\write heads that are inside the metal casing above.

The commonest type of hard disk uses the IDE bus and its performance is quite adequate for most users. 

If a higher level of performance is required, a SCSI hard disk could be chosen.  This runs much faster, but it is more expensive.