The plot roamed widely, and included quite a few science fiction elements, including mutants, aliens (with winged monkey soldiers), cyborgs, and much more, but at its heart it explored questions of race in the real world through the interaction of apes and humans (including different types of apes, and with different communities of humans, some of which were completely integrated).
One of the best things about the series, though, was the artwork by Mike Ploog, Frank Chiaramonte, Tom Sutton, Herb Trimpe, and Virgilio Redondo. It's very trippy in places, and is definitely a child of the 1970's. Boom comics, which is currently publishing several Apes-related titles, had originally announced they were going to be doing a reprint of Terror on the Planet of the Apes, but unfortunately the project was cancelled last March, apparently due to low sales. More's the pity-- I would definitely have snatched them up! Fortunately at least some of the earlier chapters are available online in various places, for those who are interested in this fascinating bit of science fiction.