What are lipoproteins?
Clusters of choloesterol, triglycerides, other lipids, surrounded by a hydrophilic protein coating.
LDL is low-density-lipoprotein, it moves choloesterol from the liver to other tissues. But if there is too much of it in the body it just starts dumping cholesterol in arteries. This is how it gets the name "bad cholesterol".
HDL helps pick up some of this cholesterol and takes it back to the liver, this is how it gets its name "good cholesterol".