Many interior design experts have used the terms curtains and drapes interchangeably over the years which is acceptable.
3 main differences:
One of the main differences between drapes and curtains is the type of material. Drapes are usually made from thicker fabrics, such as velvet, silk, or damask, and they are usually lined. These fabrics add a more formal, luxurious look and feel to a room, and the lining offers an extra layer of insulation and light-blocking. They lend a sense of quiet elegance in formal dining rooms, living rooms, or bedrooms. Fabric choices for drapes are limitless and can be pleated, solid, textured, or patterned.
2. Length and Fullness
Drapes are typically floor-length or longer. They puddle on the floor to create an elegant, flowing look that blocks out light and provides greater insulation.
Curtains are normally made to cover windows just up to the sill or to extend a few inches below it. This shorter, less full look, gives a casual, breezy charm to the room and lets in more light and air, but there is no standard length for curtains and they may also be floor length.
The functionality of drapes and curtains can also differ. Drapes are considered the better option for privacy, blocking out light and noise, and providing adequate insulation. This is because the weight and length of drapes, as well as their lining, help them fall closer to the window so there are fewer gaps for light and air to enter. This makes them perfect for bedrooms or meeting rooms.
Curtains are ideal for use in rooms where the opposite is a priority. If natural light and a gentle breeze are what you want, the lighter fabric of curtains is what you need.
Source: Better Homes & Gardens