Amorphous refers to a lack of long-range order of atoms/molecules. In the case of glass, there is a local ordering of O atoms around each Si. As you 'zoom out' it becomes apparent quite quickly that there is no long-range ordering, with the appearance of random gaps and bunches of atoms.
Wikipedia is probably a good place to start: http://en.wikipedia.org/wiki/Amorphous_solid
Yes,I just want to add some points.
In general amorphous indicated unordered arrangement(random arrangement).
Units cells(like a bricks of wall for compound in solid state) is the necessary and essential component for ordered arrangement.
The compound which are lack of this unit cell Will be called as "amorphous" in nature.
Propably in glass it may not be possible.Hence it is amorphous.