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
