The use of DNA as a genealogical tool is complementary to--not independent of--traditional genealogical research. We're all too familiar with the fact that the "paper trail" sometimes provides incomplete or conflicting information. In other cases, we may suspect two disjointed lineages are connected, but we have no documentation to support the assertion. Worse yet, there can exist "proven" lines which are actually incorrect (perhaps based on faulty assumptions, misinterpretations, etc.). In all of these cases, DNA provides an unbiased method for validating (or debunking) conclusions that are based on traditional genealogical methods.
The Y chromosome is transmitted from father to sons. Scientists have identified a small portion of the Y chromosome which is passed virtually unchanged. Testing of this portion of the Y chromosome provides information about the direct male line, which is the father, his father, and so forth back in time. The locations tested on the Y chromosome are called markers. Occasionally a mutation (i.e., a small change) occurs at one of the markers in the Y chromosome. These occasional, random mutations, are valuable for genealogists in identifying different branches of the family tree. Thus, by comparing the markers of two individuals, we can determine if the two are related. If they are related, we can then estimate how recently their most recent common ancestor (MRCA) lived.