What are the main characteristics used to differentiate between the angiosperms and gymnosperms?

Gymnosperms do not have flower whereas angiOSperms have flower

The seed of the gymnosperms is unprotected (naked seed) compared to angiOSperm's seed is protected in ovary.

Gymnosperms consists of tracheid (except Gnetum sp. ) while angiOSperms have complex vascular tissue ( develop vascular tissue)

The reproductive structure for gymnosperm is cone and the reproductive structure for angiOSperm is flower. :)