With the popularity of DNA testing kits and ancestry websites, I often hear from people wondering if their test results of "X% Ashkenazi Jewish" mean that they are Jewish.
While DNA testing can indicate the likelihood that one has Jewish genetic ancestry, that's not the same as being a Jew. Being Jewish means that one is a member of the Jewish people, or what Rabbi Mordecai Kaplan famously called a "religious civilization" - an ethnicity, culture, and faith tradition all rolled into one. Our religious and cultural identity is determined not only by our DNA or our genetic lineage.
Reform Judaism in the United States considers an individual to be a Jew if either parent was/is Jewish and the individual was raised with Jewish identity and practice, or if the person formally converted to Judaism. Someone who becomes Jewish by conversion is completely Jewish regardless of their DNA or genetic heritage.
Many people who discover that they have some genetic Jewish heritage decide to learn more about Judaism. A great place to begin that exploration is with your local Reform congregation, or by taking a class about Judaism.