Wisps are your basic construction unit, very similar the Grunts, Peons, and Acolytes. For the Night Elves, wisps perform the tasks of collecting gold, wood, and building structures.
Up to five Wisps can collect from a goldmine at any given time. If you want to remove a Wisp from the mine, you select the mine, then click an individual Wisps from the unit pane at the bottom center of the interface. In order for a Wisp to collect gold from the mine, the mine must be "Entangled". In order to entangle a mine, you must root your Tree of Life nearby. You can entangle additional goldmines by moving your Wisps to one on the map, and using it to seed an additional Tree of Life. In addition to an additional mine this creates a sub-base that you can use in the same manner as your main base.
Wisps are able to gather wood, five at a time, by circling an individual tree. Doing this, they extract wood without actually damaging the tree itself, which means it is impossible to ever run out of trees, or create gaps in your defense by over-harvesting. You will likely want around six wisps actively harvesting wood in order to keep up your stores.
One wisp may construct a single building at a time. This means that construction can be a bit on the slow side. Additionally, if you are constructing a living building, such as an Ancient of War, or Ancient of Lore, etc. the wisp will be consumed. Keep that in mind and produce additional wisps as necessary.
Wisps can be set to auto-repair nearby units and structures. For this reason, it is a good idea to have extra wisps placed throughout your base in case of an attack. To enable auto-repair, select your wisps (double-click to select All) and right click the Renew ability.
If your base, or other area where you have multiple wisps, falls under heavy attack you may decide that it is high time for a desperate move. You can maneuver your wisps into position near enemy forces and detonate them. This, of course, kills your wisps but can inflict enough damage to your opponent to buy much-needed time and turn the tide of the battle.