Heterocysts are certainly everything Webster 1913 says they are, vague that it might be. However, living in 2004, and not 1913, I can tell you a little more.
Nostoc is a very common
nitrogen fixator. Nitrogen fixing algae (all of which are
bacteria) take triple bonded
nitrogen from the air and turn it into an organically usable form, usually by combining it with
hydrogen to make compounds like
ammonia. This is very important, since there otherwise would be no way for us poor carbon-based lifeforms to access nitrogen. As well you may know, we need nitrogen for making protein. Proteins aren't just muscle -- they also
regulate chemical reactions and form our cell membranes.
So, anyway...heterocysts. Heterocysts are where nitrogen fixation takes place. They have extra-thick walls, in order to protect nitrogenase (the enzyme which helps fix nitrogen) from oxygen. Oxygen is to nitrogen fixation as the canyon valley is to a plummeting Wile E. Coyote: it stops it -- and how!
Heterocysts are nonphotosynthetic, which is a further protection from oxygen. Seen under the microscope, Nostoc resembles strings of rosary beads, with long successions of smaller, squarish cells intermittantly broken by the inclusion of larger, rounder heterocysts. Although Webster notes that heterocysts are found in Nostoc and relatives, they aren't exclusive to those bacteria. Most of the nitrogen fixators we know of possess heterocysts, whether they're closely related to Nostoc or not.