The Occitano-Romance languages are a subfamily of the Romance languages that straddles the border between Spain and France. Historically, Spanish and French linguists have been reluctant to recognise this as a legitimate subgrouping of Romance, preferring to stress the relationship of Occitano-Romance languages with the dominant language of the country in which it’s spoken, or else to claim that the entire Occitano-Romance family is part of the family that includes the national language (i.e. that it’s all a subsubgroup within Gallo-Romance or Ibero-Romance).
The namesake of this group is, clearly, Occitan, a language which was once spoken throughout the south of France and one of the most useful languages in western Europe during the Middle Ages. It’s often known by the name of one of its dialects (like Provençal, Limousin, Languedocien, Gascon). The language policy of French has long been to forbid Occitan’s use in any kind of official setting and for decades children were shamed if they dared speak it at school; as such, Occitan is now fairly moribund, mostly spoken only by elderly people in rural areas. There are language revival efforts but these are limited in the face of extremely unsupportive French policy.
Catalan is the other major language in this group, and is doing much better, with official recognition (and even preferential treatment) in the Spanish autonomous communities of Catalonia, Valencia and the Balearic Islands, 4 million native speakers, and 10 million total speakers. It is also often known by the name of its dialects (e.g. as valencià in Valencian).
Aragonese is also sometimes included this group, as it shares some characteristics in common with Occitano-Romance languages, like the presence of partitive and locative clitics (en and i) and some phonetic commonalities. Others consider it an Ibero-Romance language, and perhaps the safest bet is to consider it a transitional language variety that straddles the two.