From Envirowiki

Urine (also piss) is an animal output. It's used to get rid of excess liquids, nitrogenous wastes, and toxins from the body. It is generally sterile.

Urine is composed mainly of urea, ammonia, or uric acid, depending on what type of animal produces it.

[edit] 1 As waste

Urine is often flushed down the toilet, and treated as sewerage. Often, such waste ends up in waterways or the ocean, where it increases nutrient levels, and leads to problems such as algal blooms.

[edit] 2 As Fertiliser

Many agricultural regions are low in nitrates, either naturally (arid regions) or due to intensive farming which depletes the nitrogen from the soil. Nitrogen compounds are vital for plant growth, however plants can not absorb nitrogen from the air, and must retrieve it from the soils they grow in (some plants, such as legumes, have a symbiotic relationship with rhizobia, which can absorb nitrogen from the air, and make it usable for plants).

Often, in industrial agriculture, artificial nitrogen fertilisers - usually produced from natural gas - are added to the soil. Urine can potentially replace this fertiliser, leading to increased plant growth AND reduced carbon emission from fertiliser production, therefore reducing potential anthropogenic global warming.