LDL and HDL are both lipoproteins, particles that carry cholesterol through your bloodstream, but they do opposite jobs, which is why one gets called "bad" and the other "good."
LDL (low-density lipoprotein) carries cholesterol from your liver out to the rest of your body. The problem is that when there's too much LDL cholesterol circulating, it tends to deposit on the walls of your arteries, forming plaque. Over time, this plaque buildup narrows and stiffens the arteries, a process called atherosclerosis, which raises your risk of heart attack and stroke. That's why LDL is labeled "bad" cholesterol: high levels directly contribute to clogged arteries.
HDL (high-density lipoprotein) works in the opposite direction. It picks up excess cholesterol from your blood vessels and tissues and carries it back to the liver, where it gets processed and removed from the body. Essentially, HDL helps clean up cholesterol rather than depositing it. Higher HDL levels are associated with lower cardiovascular risk, which is why it's called "good" cholesterol.
So it's not that cholesterol itself is good or bad. Your body actually needs cholesterol for things like building cell membranes and producing hormones. What matters is which type is carrying it and in what direction. Too much LDL relative to HDL tips the balance toward arterial buildup; enough HDL helps counteract that.
A standard lipid panel reports both numbers along with total cholesterol and triglycerides. General targets are LDL under 100 mg/dL (lower if you have heart disease risk factors) and HDL above 40 mg/dL for men or above 50 mg/dL for women, though your doctor will weigh these against your overall risk profile, family history, and other factors rather than looking at any single number in isolation.
Diet, exercise, smoking status, and genetics all influence both numbers. Saturated and trans fats tend to raise LDL, while regular aerobic exercise and healthy fats (like those in olive oil and fish) tend to raise HDL.