Out of inquisitiveness people do ask about knowing something new. Since time immemorial ancient sages have revealed many secrets of this universe which are related to both the gross and the subtle world that we witness .
What the sages have discovered in their inner self is a great vast knowledge which have implication both in action related i.e. the rituals in the gross world and the knowledge part connected with the inner world. It is the Mantras which are depicting these many things. All these Mantras are in Vedas.
In those Mantras there are portions where it is spoken about our Real Self, the Consciousness, The Atman, the Brahman. these are Upanishads, they come under Vedas. It is Vedanta. Upanishads are Vedanta. Also since it come at the last of Vedas generally so they are called by the name Vedanta.
Furthermore another definition may come: there are six basic schools of philosophies in ancient Sanatana civilisation; they are i) Sankhya Darshan ii) Vaisheshik Darshan iii) Naya Darshan iv) Patanjal Darshan v) Purva Mimangsha vi) Uttar Mimangsha. This last one Uttar Mimangsha is Vedanta Darshan.
In other words we can say Vedanta actually connects us with our inner Self and thereby helps us to attain unimaginable strength in oneself. It reveals the inner divinity already in men.
Then there can be many types of Vedanta viz. Advaita Vedanta, Dvaita Vedanta, Vishisthadvaita Vedanta and so on..