The answer to this type of question can range from the general description "Decorative Lighting" up to more specific terms. Such as Christmas lights, Shop-front Lights, Pendant lights etc..
There is no one definition that fits all! The reason behind it is because based on your personal taste everyone has a different idea about what must be decorative lighting and vice versa. So far there are hundreds differences in usage but also common aspects which are always present in decorative lighting.