Triangle Centers:
G: Centroid - Intersection of the three medians of the triangle
H: Orthocenter - Intersection of the 3 lines containing the altitudes
C: Circumcenter - Point in the plane equidistant from the 3 vertices of the triangle. (determined by the perpendicular bisector of each side)
I: Incenter - Point on interior of the triangle that is equidistant from the 3 sides. (determined by the angle bisectors)
Euler Line: Line segment containing the Centroid, Orthocenter and the Circumcenter.
If Geometers Sketchpad available: GSP Triangle Centers