A stellar atmosphere (or stellar envelope) is the gas/plasma surrounding the visible star, essentially that part of the star which you can see through. The photosphere is the layer of the star which (by definition) produces the light emitted by the star, and the atmosphere is taken to be the photosphere plus nearby the gas/plasma above it, which (as seen in the Sun) glows a bit but is thin enough to be basically transparent.
The layer immediately above the Sun's photosphere is termed chromosphere, some thousands of K and a few thousand km in height. Beyond that is a very large, less-dense outer envelope termed the corona consisting of plasma on the order of a million K. The top of the corona grows thinner with height, fading into the interplanetary medium and solar wind, and varies with solar activity, but the corona clearly is on the order of millions of km in thickness. Between the chromosphere and corona is a very a narrow layer (on the order of 1000 km in height), termed the transition region, in which the temperature rises very rapidly by height.
It is presumed that Sun-like stars have much the same atmosphere and other stars have variations on this theme, but the Sun is clearly the example for which the most evidence has been collected. The lower layers of other stellar atmospheres, especially the photospheres, are much studied through the development of stellar model atmospheres aimed at matching the spectral features of EMR from the star.