Vedas refer to the scriptures compiled by Sage Ved Vyasa in very ancient times. There are four of them – Rig, Sama, Yajur & Atharva. Each Veda consists of 4 parts – mantras, brahmanas, aryanakas & upanishads. The first 3 parts are called karma kanda as they involve rituals or karma. The last part is referred to as jnana kanda and is purely philosophical, with reflections on the ultimate Truth or Brahman.
The upanishads part is also referred to as Vedanta, which translates into English as “end of knowledge”. It is concerned with elucidating the highest truth or Brahman.
The Gita consists of 700 verses and is extracted from the epic, Mahabharata. While the upanishads are terse and cryptic, the Gita is explanatory and understandable even by an new aspirant on the spiritual path. It puts forth the knowledge of life & living across its 18 chapters.
The Upanishads, Gita & Brahman sutras are also referred to as the prasthatrayi or the three authoritative texts.
Hope that helps.
