Fixed or Tracking Array
The solar array can be either fixed mounted or mounted on a sun tracking rack that follows the sun. There are two main types of track racks, one axis of rotation, or sun-tracking with two axes of rotation.
A sun tracking rack can increase the output of the solar modules because it will follow the sun throughout the day. To know whether you should consider a track rack you will need to complete a detailed cost analysis and return on investment calculation as this decision will be specific to your location and case.
In general, I personally shy away from track racks for a few reasons. First I have found from experience that the maintenance for most of these systems can be a challenge, even more so as the array and rack ages. Keep in mind solar modules have a 25 year warranty and a life expectancy in excess of 40 years. With such a long life a rack that moves will break at some point and will need to be maintained. The next main reason I shy away from a track rack is the return on investment. Solar modules have come down so much in cost that it can be hard to justify the additional rack cost and the cost of maintenance for the tracking rack over just adding a few additional modules. The ROI will vary based on your location so if you are in a location like the south west US you might see a high enough output gain to more than justify the additional costs.
Image Credit: NREL Changing System Parameters http://rredc.nrel.gov/solar/calculators/pvwatts/system.html