Yeast is available in two forms - liquid and dry.
Dry yeast is easy to use, economical, and good for general brewing although the quality is not as high as liquid yeast. Why? The process of drying yeast is almost impossible to keep totally sanitary. So when you pitch dry yeast you are also pitching whatever contaminants that may be present in the drying process.

Liquid yeast is grown in sterile conditions and is generally purer and available in more strains than dry yeast. Liquid yeast will enable you to get closer to the specific style of beer you are brewing and have more control over the flavor. You end up with a cleaner tasting beer.

The type of yeast that you use in brewing is as important as any other ingredient in your beer. Yeast is biologically classified as a fungus. It is a living microbiological organism, metabolizing, reproducing and living off the ingredients that you have concocted in order to make beer. Yeast is the thing that finally determines what the flavor of the beer will be.

What are the main types of beer yeast?
There are two main varieties of beer yeast that are used by brewers. They are classified as ale yeast ("top-fermenting" type, Saccharomyces cerevisiae) or lager yeast ("bottom-fermenting" type, Saccharomyces uvarum, formerly known as Saccharomyces carlsbergensis). These two varieties of beer yeast are further broken down into categories of specific strains. In the world of brewing today there are hundreds of strains of both ale and lager yeasts, all of which will offer variety to the finished beer.