Difference between contemplation and thought

On the one hand, we're told in Buddhism to "stop thinking", "do not think about thinking", and such things, by monks and laymen. As an example, I remember a monk describing how when he first started, his teacher would shout at monks (a simple HEY! or HO!) he suspected are deep in thought, to wake them up. On the other, there's such a thing as "contemplation of death". One is to sit and think about death. What's the difference between *thoughts* and *contemplation* in the eyes of someone who would espouse anti-thought advice?