The short answer to this question is that any religion practiced anywhere that has its origins or a signficant part of its origins among the Bantu speaking people of both Congo Republics and Northern Angola is a Kongo derived religion. It is a little more complicated in reality. Kongo derived religions in the Americas sometimes include Bantu elements that are not Kongolese at all and most Kongo derived faiths in the Americas have been influenced, sometimes very much, by Yoruban religion. Of course, Orisha worshippers have borrowed much from Kongo religion, too. Narrowly speaking, a Kongo or Kongo derived religion is one practiced in either of the Congos or among Kongo speaking peoples in neighboring Angola, as well as any religion made up of predominantly Kongo elements.
In Africa, many people in Angola, the former Zaire, and Congo Brazzaville would consider a variety of Independent Christian traditions as "Kongo religion". Kimbangism is a prime example of this kind of Kongo religion. InquiceWeb is concerned with traditional religions. Also, while strictly speaking a Bantu religion and not Kongo, many elements of traditional Shona religion would be identified by Kongo religionists as the same as their faith. For that reason and because it is a fascinating site, there is a link concerning Shona divination on the mainpage.
In there Americas there are many Kongo derived religions still being practiced today. There are two main aspects of Kongo religion that are quite distinctive. One is the practice of bringing down spirits of the dead to briefly inhabit the bodies of the faithful. The purpose of this is so that the ancestors may share their wisdom, providing spiritual assistance and advice to those here on Earth. Without exception, all such faiths in the Americas retain this central feature of Kongo faith. The other feature is the extensive work with Inquices (Enkises, Nkisi). The Inquices are very like the Orishas of Yoruban tradition, but also different. In Cuba and Brazil where Yoruban influence was strongest in the Americas, they are often syncretized with the Orishas. They may best be described as being both the most ancient of ancestors as well as being associated with specific powers in nature. The Inquices do not tend to possess as detailed a mythology as the Yoruban gods.
In Cuban Palo Mayombe, the believer possesses a Nganga. In Africa Nganga means the priest. In Palo (Las Reglas de Congo) Nganga is the term used for the ceramic pot or the iron cauldron in which the Palero or Palera keeps the spirit ally. This is the focus of most Cuban Kongo religion. In the Kongo side of Haitian Vodoun, the smaller sized Pakets Congo serve much the same function. Both are derived from an original Kongolese practice in Africa. These are sometimes very decorative and like the Nganga, may include feathers. While the Nganga usually is wrapped with chains, the Pakets Congo will have multicolored ribbons.
In Jamaica, the material elements of Kongo faith have not survived so well. The Kumina of Eastern Jamaica have however retained their drumming and dancing and still bring down the ancestors to help them. Like the Kongo practicioner from Cuba, they have kept a large amount of the Kongo language alive.
In Brasil, well everything in Brasil has its own flavor. There are several Kongo derived religions. Several types of Candomblé in Bahia are perhaps the "purist" form of Kongo tradition. Also there is, especially in the south, Umbanda based upon a mixture of Yoruban Orixa worship and a very solid Kongo base. Quimbanda is perhaps the closest form in Brasil to Palo. While they have no Nganga, other aspects of their traditions are similar. Lately Quimbandistas, who have for many years received very bad press, have been fighting to reclaim the respect they deserve.
In the USA, Kongo religion was always stronger than Yoruban. Remember Congo Square in New Orleans, and the Gullah (Angola) of the Carolina Islands? In recent years Kongo religion has begun to return to the USA. Haitian Vodoun is practiced from coast to coast. Palo has spread to Miami, New York, New Jersey, Philadelphia, and elsewhere. In California Umbanda has followers and there is Candomblé in NYC.