Certain manures can be used to great benefit in enriching garden soil and making it more fertile. Before rushing out and getting some random shit and throwing it on your flower beds there are some things you should know however.
Manures come in a variety of shapes and sizes. The characteristics of a manure are determined by what animal created the product, how long ago it was deposited, and what the manufacturer had been eating. Only manures from plant eating animals should be used in your gardening. The high fat content in the wastes of meat eating creatures creates a manure that doesn't break down well. Manures where you can see the fiber content, such as horse, llama, and rabbit are often able to be applied directly to your soil without harming the plants there, while the more homogenous manures such as cow and poultry will need to be diluted or composted. The reason for this is that the nitrogen content is often very high in the later, and plants may be 'burned' by these 'hot' manures.
Prepackaged manures such as the kind you see outside grocery stores in the spring are easy to use and get, but I do not recommend them. These mixtures contain very little actual manure. They are a composted mixture, often with a high wood chip content, and the nutrients required to complete the breakdown of the wood in the mixture often results in depleting the soil of nutrients rather than adding good stuff to the soil.
The very best manures are aged. This can be achieved in several ways. Many farmers and ranchers have a manure pile outside their barn where they stack the manure and straw and hay when they clean out their buildings. This is an excellent source of aged, high fiber manure and many times you can just back up a truck and go home with free organic fertilizer. Some farmers charge for this, but the fee is not much. The manure in these piles is often already aged, almost odor free, and ready for application on your garden beds.
Composting is probably the best use of manures. The manure is mixed in a compost pile with yard waste and plant waste from your kitchen, aged and stirred. A wonderfully rich soil results from this process. This can take anywhere from 2 weeks to a year, depending on the size of the pile, the content, and the desired soil consistency. Despite what Martha Stewart says, compost does not have to look like prepackaged potting soil to improve your garden soil. The compost can be mixed into your original garden beds, or simply spread over the top, where the nutrients will leach down to the roots of your plants.
Another great use of manure is the making of manure tea or compost tea, but those processes are written up in other nodes, so I won't go into great detail here.