Viewpoint
View points are any structure high enough from where it's possible to observe the surrounding terrain. They range from church towers and mansions' roof beams in the cities to watchtowers and guardposts on the roads.
Assassin's Creed
In the first game, view points were found in all the cities (Damascus, Jerusalem and Acre) and in the Kingdom, excluding Masyaf. Each map contained 10 to 12 view points located on the highest buildings; such as a cross on top of a Catholic Cathedral and the top of a lighthouse on the port-city of Acre, or a Mosque's roof and a palace guardtower on the city of Damascus. Aside from the height of those constructions, they could be located by watching the birds that there set nest. Climbing atop one of those provided the means to activate the view point via Altair's Eagle Vision.
Those view points allowed Altaïr to view the layout of the terrain around him and watch the movement of its people. After the first view point in a map was "activated", it revealed the location of the remaining points and the missions within a certain radius.
Some viewpoints had archers defending the position, or guards set around and below it. Those could fire either arrows or rocks to prevent a climber from reaching the top. All the view points had carts of hay on the ground at the very bottom, providing perfect and spetacular locations for the performance of a incredible-but-potentially-fatal Leap of Faith.
In the Xbox 360 version, synchronizing with all View Points earns you the achievement of "Fearless".
Assassin's Creed II
Assassin's Creed 2's view points function much the same and are located on famous land marks in Italy. Ezio will be able to dive into carts and stacks of hay like the first game, but also in carts of flower pedals, leaves and bodies of water. Ezio will be able to grab guards standing near railings and pull them over, stabbing and sending them falling to their death.