The Selous is one of the largest protected areas in Africa and has been a UNESCO World Heritage Site since 1982. In 2019 its northern portion — the section historically used for photographic safaris — was gazetted separately as Nyerere National Park, so visitors will see both names in use.
What distinguishes it is water. The Rufiji River spreads into a network of channels and lakes, and boat safaris here are genuinely excellent: hippo, crocodile, elephant coming down to drink, and skimmers working the sandbanks. Walking safaris with an armed ranger are also permitted, which they are not in most northern-circuit parks.
Game viewing is good rather than dense by Serengeti standards, but the combination of boat, vehicle and foot — and the near-absence of other visitors — makes it a different kind of safari rather than a lesser one.