When you create, move, or resize a slice, you can turn on snapping so that the boundary lines of your slice snap to (or pull towards) existing slice boundary lines. This is useful if you are trying to create slices of your artwork that cover every pixel without any space between the slices. For example, if your artwork represents a user interface where the position of the objects must stay the same when the sliced objects are imported into a program such as Microsoft Expression Blend, slice snapping makes sure that you create slices around those objects that account for every pixel.
On the View menu, click Snap to Guides to make a check mark appear next to the menu item.
When you create a new slice, the slice boundary snaps into place next to any existing slice boundary lines.
When you resize slices, the slice boundaries turn red when they snap to another slice boundary.
When you move slices, snapping icons appear around common corners.