I will try to help:

A packer is a downhole tool used to isolate the tubing from the casing. There are various types of packers- permanent ( which are milled up by tools like the metal munche ) and retrievable ones that can be run and out of the well.  They are also set by wireline, mechanically , and hydraulically. Check out www.teamoiltools.com for pictures of packers.

Roughly Skin is a term used to describe the flowabilty interface of the well formation. The important thing is that the Service companies do not want to damage the skin and reduce the flow of the formation.  

Is the tubing inside the packer?. I saw thw pictures of the packers but I am not clear how is the asembly between the tubing and packer.

The packer bore is a continuation of the tubing. Usually the tubing is connected to the packer with an oilfield thread ( very similar to NPT threads but there are special ones). Most packers have a female thread on top and a male thread on bottom- just like the tubing joints.  Packers usually have an id that is compatable with tubing id.

Look at the AS1-X packer drawing at Team's web site . The top of the drawing is a top sub with female threads while the bottom of the drawing is the bottom sub with a male thread.

Hope this helps

Please can someone help wit the formula for computing skin factor from your sg, flp, etc

Calculating Skin isn't that easy.  I simple definition for skin is "fudge" factor to make well performance act like the theoretical model.  Skin is either calculated from production analysis, build up analysis, or in reservoir simulation.  It requires some type of well performance data (production and pressure data over time).

Hope this helps.