The Samhitas are the core of the Vedas. These consist of all the prayers, hymns and rituals.
Brahmanas are the commentary on the Vedas. These explain the social and religious importance of each ritual.
Aranyakas are the concluding part of the Brahmanas. These are the texts in forests.
Upanishads are the philosophical commentaries on the Vedas which deal with the doctrines of Karma, Moksha, Maya, Brahman and Atman.