The Vedas are the oldest extant Indian scriptures. They are a collection of hymns which were compiled by rishis, 'seers', thousands of years ago; it is said they are at least 5,000 years old. Which is fascinating since one of our favorite mantras, the Gayatri, comes from the oldest veda - The Rig-Veda - and is still a popular hit :) !
There are four vedas in total: Rig-Veda, Yajur-Veda, Sama-Veda and Atharva-Veda. They are full of symbolism and mythology and written in Vedic Sanskrit, therefore very difficult to comprehend.
The Upanishads are bodies of work dedicated to elaborating the Truths revealed. Upanishad means "to sit down near" because these are teachings said to be taught to disciples by their teachers. There are over 200 hundred Upanishads; the oldest were composed almost 4,000 years ago and the youngest are from this century.