When creating a blender-model using meshes like the cube you often get sharp edges which looks not really nice. Blender offers the great bevel-feature which you can use to round these edges to get nice soften edges for your model.
Here is a simple example of a cube which is enhanced using the bevel-operator:
Using the bevel is very easy: Select the edges you want to bevel and press the Ctrl + B keys. After that you can set the number of sgments you want to add to the edge and the size/offset of the bevel-area using the bevel-panel:
A faster method is using the mouse (move up or down) to set the size of the bevel-area and the number of segments (mouse-wheel) after you pressed CTRL + B.
When you check the option Vertex Only in the bevel-panel then just the selected vertices are beveled. For your cube this means that you get round corners. You can activate this vertex-bevel tool by pressing Ctrl + Shift + B.