Both epidermoid and pilar cysts are smooth round lumps which you can see and feel just beneath the skin surface. They are very common. Often they are small (pea size) but sometimes they slowly get bigger over many months to become a few centimetres in diameter. They look very similar to each other but can be distinguished from each other if the cells that form the cyst sac are looked at under the microscope.
There are many types of skin cysts; the most prevalent are epidermoid, pilar or sebaceous, which may become problematic if they get infected. http://articles.mercola.com/sites/articles/archive/2016/08/29/sebaceous-skin-cyst.aspx