Crown thinning is used to reduce the risk of storm damage and some pest infestations or disease. By removing branches from the outer edge of the tree it allows more light into the centre of the tree making it stronger. On more mature trees this technique will reduce the weight of the branches and on younger trees it will slow down the growth rate.
Crown reduction would be used when a tree has become too large and is presenting a danger to surrounding buildings. Some species are not suitable for this process, which can cause rapid decay of the tree. If there is a lot of reduction required, then it should be done in two stages to avoid tree damage.