This article is from the Storms FAQ, by Chris Landsea firstname.lastname@example.org with numerous contributions by others.
The tropical cyclone is a low-pressure system which derives its energy
primarily from evaporation from the sea in the presence of high winds and
lowered surface pressure and the associated condensation in convective
clouds concentrated near its center (Holland 1993). Mid-latitude storms
(low pressure systems with associated cold fronts, warm fronts, and
occluded fronts) primarily get their energy from the horizontal temperature
gradients that exist in the atmosphere.
Structurally, tropical cyclones have their strongest winds near the
earth's surface (a consequence of being "warm-core" in the troposphere),
while mid-latitude storms have their strongest winds near the tropopause
(a consequence of being "warm-core" in the stratosphere and "cold-core"
in the troposphere). "Warm-core" refers to being relatively warmer than
the environment at the same pressure surface ("pressure surfaces" are simply
another way to measure height or altitude).