What about them?

Just think of them as:

All cells begin unspecialised with all genes 'switched on' 

Differentiation (or specialising) is when certain genes switch off

Each gene codes for a differenent protein (one gene one protein theory), so a hair cell for example will have specialised to only produce the proteins it needs e.g keratin 

Unspecialised cells in plants are called meristems, in animals they are called embryonic stem cells and we only have them up to the 8 cell stage

There are also part specialised cells such as bone marrow that can only change into a few different types of cells

Hope this helps! :)

Joe