endoskeleton vs skeletal system

endoskeleton and skeletal system both are nouns.

Word	Noun	Adjective	Verb	Adverb
endoskeleton	Yes	No	No	No
skeletal system	Yes	No	No	No

As nouns, skeletal system is a hypernym of endoskeleton; that is, skeletal system is a word with a broader meaning than endoskeleton:

endoskeleton: the internal skeleton; bony and cartilaginous structure (especially of vertebrates)
skeletal system: the hard structure (bones and cartilages) that provides a frame for the body of an animal

Other hypernyms of endoskeleton include frame, skeleton, systema skeletale.

endoskeleton (noun)	skeletal system (noun)
the internal skeleton; bony and cartilaginous structure (especially of vertebrates)	the hard structure (bones and cartilages) that provides a frame for the body of an animal