Before you study about what is a triangle you are advised to read
What is a Polygon ?
"Quad" means Four, so a polygon which have 4 Angles, 4 Sides and 4 Vertices is known as Quadrilateral.
In the Above diagram : Quadrilateral ABCD has 4 Sides  AB, BC, CD and DA 4 Angles  ABC, BCD, CDA and DAB 4 Vertices  A, B, C and D
In the Above diagram : Quadrilateral PQRS has 4 Sides  PQ, QR, RS and SP 4 Angles  PQR, QRS, RSP and SPQ 4 Vertices  P, Q, R and S

