What is cholesterol? What is it's physiological function? Should we care?

Question 1: What is cholesterol?
Question 2: How does it affect the human health?

Most laypersons would answer both questions together, and say "cholesterol is the thing that causes high blood pressure, heart disease, obesity, etc."

To me, the layperson has answered the 2nd question, but did not answer the 1st question. More specifically, the answer above answered the question "what is hypercholesterolemia".

For a chemist, the definition of cholesterol is something that can only be answered by drawing, and not by words. Therefore, according to any organic chemistry textbook, cholesterol is defined structurally as the image seen above.

Therefore,
Question 1: What is cholesterol?
SYC: See the picture above.

Question 2: How does it affect human health?
SYC: An excess of cholesterol causes hypercholesterolemia (heart disease, high blood pressure, etc.)

Consider a third question:
Question 3: What is the difference between bad-cholesterol and good-cholesterol?

The answer depends on whether one is asking a nutritionist or a chemist.

As a chemist, I would describe bad-cholesterol using drawing as [see the image above], and good-cholesterol as [see the image above], therefore, they are one and the same.

A nutritionist would answer differently. The following is answer is taken from Wikipedia.
LDL particles are often termed "bad cholesterol" because they have been linked to atheroma formation. On the other hand, high concentrations of functional HDL, which can remove cholesterol from cells and atheroma, offer protection and are sometimes referred to colloquially as "good cholesterol."

Finally,
Question 4: What can/should I do about my health/lifestyle to treat/prevent hypercholesterolemia?
SYC: I'm not a nutritionist. General knowledge suggests physical exercise and healthy diet. Contact your physician or nutritionist.