Why my rigging doesn't work? I just begin in Blender, so be indulgent. 
So my problem is that when I want to move my bones in pose mode, my mesh become slack and react just like I deplace one edge in edit mode. What should I do ? Did I do some mistakes ? I don't know if it is important but my mesh come from Makehuman with the armature. 







Just look stupid...
Problem with the weight Paint, probably a to low resolution, give it a subsurf modifier (just in Addition) and then, because you are making something natural ( a Body) I would always use weight paint to tell the bones which area they should affect (just optimizing the weight paint should actually work already)