Marking timber or selective harvest is only one method of harvesting timber, but is more involved than most. Depending on the objectives you have for your land, the age/size of your timber, tract size and your financial goals, marking timber might be a solution.
Prior to marking it is highly advisable to have a current inventory on your property that shows the breakdown of species, sizes and volume. This will allow you to determine how to structure the marking operation.
Selective harvest can done in pine or hardwood stands to accomplish several objectives. In hardwood stands, marking to leave "crop" trees for a future harvest is quite common. In this operation those trees that might be over-mature, mis-shapen or less valuable species can be removed and those trees left are of good form, high value species and sufficient size.
The same is true for pine stands. Typically these stands are marked to leave a particular spacing of the residual trees. This is done to maximize their growth rate and remove inferior trees.