New Caledonia is the only French local government that is not a collectivité territoriale. It has its own articles in the French constitution. Since it cannot be categorized, it sometimes unofficially is called a collectivité sui generis, but "collectivity" is not, strictly speaking, a legal category. It is sometimes unofficially called a pays (d'outre-mer) because its local legislative assembly (the congress) can rule using its own lois du pays.
Corsica is the only collectivité territoriale that is within metropolitan France.
The régions are divided into départements: ROMs are divided into DOMs; New Caledonia is divided into provinces; départements, COMs (except Saint Barthélemy, Saint-Martin, and Wallis and Futuna), DOMs, and provinces all are divided into communes.
Paris, French Guiana, Guadeloupe, Martinique, Mayotte, and La Réunion are the only territorial entities belonging to two categories. Paris is both a département and a commune, and the others are both ROM and DOM. Paris therefore has one single president and mayor and one single assembly, while ROM / DOM each, except Mayotte, have two presidents and two assemblies, although Martinique and French Guiana have a fused, singular assembly from January 2016.
Each COM has its own statutory law that gives it a particular designation (French Polynesia is designated as a pays d'outre-mer, Saint Barthélemy and Saint-Martin as collectivités, Saint Pierre and Miquelon as a collectivité territoriale, and Wallis and Futuna as a territoire).
The category of overseas territory (territoires d'outre-mer) was eliminated under the constitutional reform of 28 March 2003. French Southern Territories is still a TOM, but this is now a particular designation, not a category. This uninhabited territory no longer is a collectivité territoriale.
Mayotte and Saint Pierre and Miquelon used to be collectivités territoriales belonging to no category (but with a status close to that of a DOM), sometimes unofficially called collectivité territoriale à statut particulier, or collectivité territoriale d'outre-mer.
Mayotte held a vote in 2009 to change its status, and became a ROM in 2011.
New Caledonia will vote between 2014 and 2019 to change its status, and it may become a COM.