gluLookAt

I tried
http://www.opengl.org/documentation/specs/man_pages/hardcopy/GL/html/glu/lookat.html
Link but theres
now new problem
What is UP vector
where its direction
some where i seen it as
0(upX), 1(upY), 0(upZ)
on surfing
Q but cant understand where and how to point this UP Vector