Traditionally pollarding was used as a way of growing timber with a view to periodically harvesting it and is basically a form of coppicing, the difference being with pollarded coppice the harvested timber is kept out of the reach of browsing animals. Regular pollarding can prolong a trees life, but it means that a strict management regime is started when the tree is young and most importantly adhered to, whereby it is re-pollarded periodically. The tree will, in time, form what are called "pollard heads".
A word of note - mature trees should not be pollarded as a crude means of size control. This is termed lopping & topping and goes against current best practice and will ultimately make the tree more hazardous and costly to maintain long-term. In this instance, it is probably better to remove the tree and re-plant with a more suitable species that will not require such drastic action. There are a few species however where a heavy reduction will be tolerated when mature (eg Willow & Poplar species ) but must be re-cut periodically due to their vigorous nature and weak re-growth.