Outdoor is usually used as an adjective to describe the area in which something is happening e.g. I am having an outdoor party.

Outdoors can be used as a noun or an adverb. When used as an adverb it is usually when describing an activity happening in a place that is not near houses e.g. The weather was warm enough to eat outdoors. The noun form is usually used with an adjective such as great e.g. He walked through the great outdoors.