Flan, as made in Spain and other Spanish-speaking countries is a milk-based sweet custard topped with caramel sauce. This is sometimes referred to by English speakers as a “leche flan” (“milk flan”).
This version is also made in the Philippines and Mexico, as well as in Portugal. It is basically a giant crème caramel, baked in a mould, and turned out for serving. It is made with eggs and milk. The standard flavouring is vanilla, but may include nuts such as almonds or pistachios, or lemon. You can also buy powdered mixes made by companies such as Goya.
The custard mixture is made and set aside. The flan pan (referred to in English as a “flan mould“) is heated with some sugar and water in it until the sugar caramelizes, then the custard is poured on top, and cooked. It can be cooked over top a double boiler, or in the oven sitting in a water bath.
When cooked and cooled, it is turned out on a plate, and cut into wedges to serve, with each piece getting some of the caramel sauce.