By writing $\ce{AB.xCD}$ chemists mean that there are CDs are found in the crystalline framework of AB. The most common example of this is water trapped inside the crystal structure of ionic compounds. (See [water of crystallization in wikipedia](http://en.wikipedia.org/wiki/Water_of_crystallization))

An example that's often taught is $\ce{CuSO4.5H2O}$. See that 5 that's a representative of $x$? It means that water isn't directly bonded, but it exists in the structure in a stoichiometric ratio. 
![][1]

[Source](http://202.116.45.198/wjhx/wjhx03/ziliaoku/xingzhi/1B/images/know_35.gif)

Also, [this article](http://www.che.uc.edu/Jensen/W.%20B.%20Jensen/Reprints/134.%20Dots.pdf) shows how the use of dot has differed through history among chemists. (Though it's better if you avoid what it has written about the current use of dot)


  [1]: http://202.116.45.198/wjhx/wjhx03/ziliaoku/xingzhi/1B/images/know_35.gif