Oomycota is a phylum of kingdom Chromista that includes water molds and downy mildews. Oomycetes are different from most Chromists in some respects, but molecular, biochemical, and ultrastructural studies have shown that they belong in Chromista. Unlike most Chromists, Oomycetes are heterotrophic (i.e. non-photosynthetic). They acquire nutrition in a similar manner to organisms in the kingdom Fungi. Their filaments absorb nutrition from surrounding water, soil, and decaying matter. Some Oomycetes are parasitic--they absorb nutrition from living organisms. Parasitic Oomycetes have had disasterous indirect consequences to humans.
The Oomycota are responsible for the Irish potato famine in the 1840s. Phytophthora infestans, an Oomycete which causes fatal blight of potatoes, made its way from North America to Ireland, which was completely dependent on potatoes as a food source. This resulted in the deaths of almost a million Irish people. Plasmopara viticola, or downy mildew of grapes, almost destroyed French vineyards in the 1870s. Fortunately, the so-called Bordeaux mixture of lime and copper sulfate saved the grapes.
The Oomycetes got their name (which means "Egg fungi") from their oogamous sexual reproduction. Their non-motile female gametes are contained in large oogania, and their male gametes are smaller motile sperm.
All information taken from http://www.ucmp.berkeley.edu/chromista/oomycota.html