What is the difference between a dais and a podium?

A dais is generally a platform higher than floor level enabling people seated or standing on the dais to be seen by the rest of the people in the room, often upon which is set a "head table" and chairs.

A podium is a raised platform (similar to dais).

Strictly speaking, if one person is standing on a raised platform it's a podium, if there's more than one, it's a dais.

Additionally, a 'lectern' is a desk or stand w/ a slanted top used by public speakers. The lectern is placed on the podium.