From the manual:
Worms combines the best elements from the very best games ever created. It requires great thought, strategy and elements of sheer outrageous fortune. It provides the players with an almost infinite range of playing possibilities and we guarantee that no two games will ever be the same!
Think of a landscape, any landscape. Grab some platoons of little pink Worms and liberally scatter. Give them weaponry, tools and an eye for their enemy. The aim is to ensure that you are the last team standing. Take no prisoners!
Teams take it in turns to bombard the enemy with whatever weapon they feel is likely to reap the best reward. Some weapons are limited in terms of supplies and therefore strategy is required for the best results.