From BlenderWiki

Snap

⇧ ShiftS pops-up the Snap menu (also available in the Object or equivalent menus in Edit mode). Its is a way to move the selection to a given point (first three options), or the 3D cursor to a given point (last three options). Do not mistake this snapping with the “transformation” one (see next page).

• Selection -> Grid: Snaps the selection center to the nearest grid intersection (view dependent, only affects two coordinates). Note it do not “deform” the selection (i.e. all elements are evenly translated).
• Selection -> Cursor: Snaps each selected elements to the 3D cursor position.
• Selection -> Center: Snaps each selected elements to the center of the selection (hence, it only has an effect when at least two elements are selected).
• Cursor -> Selection: Snaps the 3D cursor to the center of the current selection.
• Cursor -> Grid: Snaps the 3D cursor to the nearest grid intersection (here again, only in “2D”).
• Cursor -> Active: Snaps the 3D cursor to the active element.