A guild can be thought of as a miniature self sufficient forest. The member plants of the guild each provide some functions that benefit the guild. The plant members also then take advantage of what the guild is producing in order to benefit themselves.
By planting various plants together you can end up with a community in which all the needs of the plants are supplied by the guild itself. The end goal is that you have a garden that requires no ongoing human maintenance or resource inputs.
Each guild is generally going to have:
The centerpiece:
The centerpiece of a guild is the primary plant you're wanting to grow. You might build a guild around a fruit tree, or a black walnut, an oak tree, or a stand of asparagus. In some guilds it may be that the centerpiece is not the primary purpose. There are also guilds without a centerpiece. An example of this would be the so-called Three Sisters, where corn, squash and beans work together in a symbiotic relationship.
Ground covers:
Grasses and many weed are harmful to many trees. They compete for resources and often secrete chemicals that inhibit growth of their competitors. So, we plant groundcovers like daffodils, garlic chives, camas, and strawberries around our trees and this helps keep out the grasses. Many of these groundcovers have extra benefits. Daffodils, for example, repel deer, while strawberries provide an additional variety of fruit for you to eat.
Mulch plants:
Lots of plants can be grown specifically to be cut down so their foliage can be used as mulch. Comfrey, sunroots and lamb's quarters are plants that produce an abundance of foliage. You can grow them right under your tree and "chop and drop" them right where they grow.
Nutrient accumulators:
There are many plants that reach deep into the earth and mine for important nutrients, bringing those nutrients into their leaves and making them available to their neighboring plants. Chicory, dandelion, yarrow, plantain, borage and sunroots are just a few examples of these important plants. When adding the function of mulch along with nutrient accumulation, you end up with a powerful combintion.
Nitrogen accumulators:
Along the lines of nutrient accumulation comes the nitrogen fixers. Legumes and a few other kinds of plants have the ability to host certain bacteria on their roots. This bacteria takes nitrogen from the air and converts it into a form of nitrogen that is then available to the neighboring plants. Growing nitrogen fixers is an important part of any garden.
Fruit trees need pollination, and some plants are especially good at attracting our buzzing friends. Bee balm, fennel, dill and borage are the first ones I think of in this category.
Pest repelling plants:
Pests are a fact of life, but there are strategies that can be employed to defend against them. Certain plants can confuse or repel pests. Nasturtiums and certain marigolds belong in every garden.
My plan is to write articles describing all the various guilds that I'm growing, along with those that I'd like to grow.
If you want to read more about the subject, though, guilds are best described in the fantastic book "Gaia's Garden". If you don't have that book, you would do very well to buy it.