Beer styles help to define the beer that we are drinking. Understanding beer styles will give one a better idea of the characteristics of a particular beer: visually, aroma, taste, mouthfeel, and overall. These guidelines should be used when reviewing a beer and deciding if you like it or not. There is always the first impression of a beer but if it is made with a clear understanding of style, the beer can be enjoyed for what the brewer intended.
Beer styles are an ever changing and ever evolving list. American craft brewers can be thought of as artists and, like any great artist, the outstanding brewers will define and redefine a style based on their perception. So beer styles should evolve in order to keep the American craft beer industry progressing forward.
The Beer Judge Certification Program (BJCP) is a great way to become more involved with beer and to truly appreciate American craft beer. The BJCP was founded in 1985 to promote beer literacy, appreciation of real beer, and recognize beer tasting and evalution skills (from their site). An examination is given in order to receive certification.