What are the differences between a crater and a caldera?It seems that caldera is a larger depression than a crater. But it is hard to recognize...

In strictly volcanic terms, they are two different kinds
of depressions.  A caldera is formed when a large eruption of magma, or lava, leaves a
gigantic empty chamber underground.  The volcanic material above the chamber, usually
made of volcanic rock and tuff, collapses into the empty magma
chamber.

Yellowstone National Park is mostly a giant
caldera, as is the Valles Grande caldera in New Mexico and others--remains of giant
eruptions in the past.

A crater, on the other hand, is
almost always a vent for volcanic activity.  Large volcanoes form when magma and other
material is ejected from a vent, which builds up a cone around the vent, and as more
material vents over time, a mountain is eventually
formed.

Take Mt. St. Helens in Washington State, for
example, the most recent eruption we have to study.  The eruption literally blew away
the top 1800 feet of the mountain, leaving a large crater.  It is not the result of
magma being displaced and then the land above collapsing in on itself, but rather, an
explosive eruption.  There is a large lava dome in the center of the crater as the vent
there continues to do what it has always done, eject magma.