Before you study about what is a triangle, you are advised to read
What is a Polygon ?
"Tri" means Three, so a polygon which have 3 angles is known as Triangle. A Triangle have 3 Sides, 3 Angles and 3 Vertices. In order words: Triangle is a simple closed curved made up of three line segments.
In the Above diagram : Triangle ABC has 3 Sides  AB, BC and CA 3 Angles  ABC, BCA and CAB 3 Vertices  A, B and C
In the Above diagram : Triangle PQR has 3 Sides  PQ, QR and RP 3 Angles  PQR, QRP and RPQ 3 Vertices  P, Q and R

