When using the Move Tool and various creation tools, you can snap to existing objects in the scene.
To snap a move, 
-click the object you want to snap to:
| To snap to... | Hold | Or turn on this icon in the Status Line (toolbar) |
|---|---|---|
|
Grid intersections |
x |
 |
|
Curves |
c |
 |
|
CV, vertex, or pivot |
v |
 |
|
Center of a geometry |
 |
|
|
View plane |
 |
|
|
Surface |
 |
Note:
If you have snapping turned on and drag an arrow on the Move manipulator (as opposed to the center), the manipulator snaps to the first available point along that axis.