In summary, a taxonomy is the practice and science of classification.
In terms of search, we focus on classifying keywords into three distinct classes - navigational, informational and transactional.
If you can determine user intent behind keyword queries, you can better target your keyword strategies. For example, if your aim is to sell goods online, you may choose to focus on transactional queries e.g. "where can I buy an LCD monitor....", as opposed to informational queries e.g. "power requirements of an LCD monitor......".
There is, of course, a lot of cross-over between these three types of queries, which