snap edge to edge in edit mode Pls see my attachment , i'm in edit mode and i want to move the selected edge along the red arrow direction so the edges will perfectly snap and overlap to target edge ( vertex A will snap to B). I tried using snap set to vertex and closest and doesn't work. Any idea how to do this ?
Here is one way: switch to vertex mode, select the vertex you want to move, and then edge slide (GKEY GKEY) it until it is adjacent to the node.  You now have a doubled vertex.  Select both vertices and merge them (ALT-M -> merge at center).  Alternately you can enable Auto merge before you edge slide.