What is the cause of acne? What are pimples?

Acne is the general term for plugged pores, pimples, and deeper lumps that can occur on the face, neck, upper back, chest, or shoulders.It is a skin condition that occurs when the oil-secreting glands in the skin are clogged and become inflamed or infected. That cause of acne

Let's know more about what is cause of acne below:
A normal follicle looks like this:

For reasons no one completely understands, follicles, often called pores, sometimes get blocked. Sebum (oil) which normally drains to the surface gets blocked and bacteria begins to grow. Both whiteheads and blackheads start out as a "microcomedone". The picture below is a "microcomedone":

Microcomedones become skin blemishes called comedones--either a whitehead or a blackhead:

Whitehead	Blackhead
When the trapped sebum and bacteria stay below the skin surface, a whitehead is formed.	A blackhead occurs when the trapped sebum and bacteria partially open to the surface and turn black due to melanin, the skin's pigment. Blackheads can last for a long time because the contents very slowly drain to the surface.

Either way, blackheads and whiteheads are filled with trapped sebum and bacteria.