I'll beg to differ with !Roxy!. An eye with a drooping lid is an eye with a ptosis - the elevator muscle of the top lid is lacking in function.
What most people call a lazy eye is an amblyopic eye. It can commonly be a squinting eye.
This eye generally has markedly worse vision than the fellow eye. When both eyes are open, the brain ignores the image seen by the lazy eye because it is much less clear than the image perceived by the good eye. It does this to avoid confusion - the brain adds up the two pictures from the eyes, (which come from slightly different angles) to form a stereoscopic or 3D image. If one image is all blurry, you can't make a clear 3D picture!
A lazy eye can be avoided by having any visual problems like an squint (strabismus) or a high spectacle prescription corrected early in childhood, generally before the age of 7. The visual system is quite plastic up until then, and can be manipulated. After that age roughly, the brain has more or less cemented what can be achieved by each eye. So if you have kids, take them for an eye check before they're 7!