From Envirowiki

Drought is the absence of rain for extended periods. Most regions have drought cycles, with semi-regular minor droughts, and infrequent major droughts, which can last years or even decades.

Droughts often have an immediate impact on industrial agriculture, which relies on a constant input of water. Modern agriculture often pushes the limits of it's local environmental support system, and uses a large percentage of the available water even in wet years, so a severe drought can have a catastrophic impact on such farming regions.

In worse droughts, ecosystems can be affected, although this is less likely, and often less severe, since all natural ecosystems have evolved over thousands, if not millions of years, and have adapted to the local drought cycles.

Humans can have an impact indirectly on drought cycles. Deforestation, which can dry out the land in the immediate area, reducing rainfall, and possibly increasing evaporation of water from the system.

Drought cycles may get worse in some areas due to anthropogenic climate change. This will cause even greater impacts to agriculture. Due to drought cycles happening outside the normal range, it could also cause severe impacts on ecosystems, and individual species, sometimes to the point of extinction.