Dermatologist Dr Nicola Ralph says…

A spot is essentially a combination of blocked hair follicles and dead skin cells. Bacteria causes inflammation below the skin’s surface and this may result in pain and discomfort. 

Blackheads: These are blocked hair follicles which may be “open” or “closed”. An open blackhead is visible as a small black dot while a closed blackhead has a tiny layer of skin over it resulting in a small skin-coloured/white bump under the skin’s surface. 
Pustules: Otherwise known as pus bumps, these are small, raised areas with white/yellow centre containing dead skin cells and bacteria and which may be painful.
Papules (red bumps): These are small raised, red areas of the skin which are often painful.
Nodules: These are large, firm/hard areas under the skin, which are also often painful.