Ruby and Tawny are both Port wines. The difference is found in the amount of time both has spent aging in casks prior to blending and bottling.
Ruby Port is younger, has spent less time in the cask, perhaps only a few years, and has retained more of its natural color, and sweet, fruity characteristics from the grapes. As a result, its colors are more of a deep, ruby color.
Tawny Port has aged longer in the cask, sometimes as long as 20 years, and as it matures, more of its color fades to a brownish, tawny color. In addition, its flavors are less sweet, have have deeper, more complex, characteristics.