What is the difference between bottom$-$up and top$-$down estimates of resources needed to complete a project?

Bottom$-$up estimates are taken from the most fine$-$grained specification of tasks and then aggregated upward; top$-$down estimates are taken from similar tasks completed on other projects.

Bottom$-$up estimates start off assuming a worst case scenario and build up from there; top$-$down estimates are based on the best case scenario.

Bottom$-$up estimates are more detailed resource estimates; top$-$down estimates try to provide more global $-$ and therefore less detailed $-$ estimates of resource requirements.

Bottom$-$up estimates start by obtaining separate information from each team member; top$-$down estimates start instead by selecting a team member who is most qualified in providing resource estimates.