Essentially a worm tower is an in-garden worm farm that allows the worms and their nutrients to interact directly with the surrounding garden bed. It consists of a vertical pipe, placed half-submerged in a garden bed, with holes drilled in it.
The pipe contains a bunch of compost worms, and you periodically feed the worms with handfuls of organic matter (kitchen scraps, leaf litter, weeds, etc).
The worms do their wonderful wormy thing and convert that organic matter into rich worm poo and worm juice. The worm juice leaches out the holes and into the surrounding garden, bringing increased soil moisture, microbiology, fertility and in turn yummy vegies, for very little effort on your part.
Source : How to make a Worm Tower