Scaling: What’s the difference between Ctrl+A and S? I saw some blender tutorials, and a doubt arises to me. Whats the difference between using Ctrl +  A and  S to scale something?
For instance, I modeled some branches using Skin modifier and if I select a vertex and use Ctrl +  A it allows me to scale but not using the  S key.
CTRLA while in Edit Mode changes the vertex radius; nothing else.
Only things like modifiers that use that property of the vertex to change something are affected. It's a nondestructive and indirect way of control since it only affects functions that read that value from the vertex.
Scaling on the other hand, at least in Edit Mode actually changes the mesh. It moves stuff to or away from the current pivot point. It is direct manipulation of the mesh.