Texture showing unwanted seams

[Justanother1]

Seams are where your model is Cut to fit on the Texture image. I can think of several ways a cube can be laid out, but I'm guessing you've cut all 6 sides into single polygons and auto-UV didn't arrange them in order. For a better UV on a cube, try cutting 3 edges on top, the same 3 edges on bottom, and one edge that connects one end, then use unfold in auto-UV. This should result in a T-shape.