This article is from the Beer and Other Brewing FAQ, by Kurt Swanson firstname.lastname@example.org with numerous contributions by others.
"Wyeast" is a nickname for the Brewer's Choice line of liquid brewing
yeasts from Logsdon's Wyeast Laboratories. There are more than a dozen
varieties of ale and lager yeasts available from Wyeast. Many brewers
that use Wyeast consider it to be of high quality, uncontaminated by
bacteria. For a report on contaminants in liquid and dry yeasts
available to homebrewers, see the "Yeast" special issue of Zymurgy.
Good results can be obtained from either dry or liquid yeasts,
especially for brewers that are willing to carefully home culture
yeasts that they know to be pure and provide good results.
The name Wyeast is pronounced like "Why-yeast", not "double-u yeast",
and is the name that the local Native Americans had given to Mt. Hood
in Oregon, which stands near the site of the Wyeast lab.