Let's start with something they both have in common: the aim is to prevent the dough from sticking during baking, and to facilitate removal from the mould afterwards.
To avoid this, you need to coat all the sides of the mold with a thin layer, or film, of fat before pouring in the dough.
During baking, this film will remain in place, reducing or even completely preventing the dough from sticking to the mold.
The difference between greasing and buttering a pastry mould lies essentially in the nature of the product used, and sometimes in the method.
Buttering
"Beurrer" means to apply butter, usually at room temperature, to the sides and bottom of a mold, to create a thin, greasy layer that prevents the dough from sticking.
Buttering is often the traditional method, especially for metal molds, and can sometimes be accompanied by a dusting of flour to further improve demolding.
Grease
"Greasing" a mold means applying a more varied fatty substance, such as butter, oil or cooking spray, for the same non-stick function.
Grease is a more generic term that includes butter as well as vegetable alternatives or oils, used according to the type of mold (metal, silicone, etc.) and recipe.
Silicone molds in particular, although often non-stick, can sometimes benefit from a light greasing, especially on first use or for sweet or rich recipes likely to stick.
In a nutshell:
- Butter = to apply butter (often soft, not melted).
- Grease = apply a fatty substance (butter, oil, spray, etc.).
Both are designed to prevent the dough from sticking, and to facilitate unmolding.
The choice between the two depends on the type of mold, the recipe, and usage preferences.
These are closely related practices, but "grease" is a broader term that encompasses "butter" among other fat options for preparing a pastry mould.