Reform Judaism is a movement within Judaism that is much more liberal and "modern" than other types of Judaism. It was started in Germany in the early to mid 1800s.
Reform Judaism aims to maintain the spiritual and moral teachings of Judaism but to dispense with the detailed laws that were such a central part of traditional Judaism. For example, Reform Judaism did away with the requirement to keep kosher. It allowed men and women to sit together at worship and did not require the wearing of traditional clothing. It also allowed worship to be conducted in the language of the country where the worshipers lived instead of in Hebrew.
Reform Judaism was meant to be more modern and to allow its adherents to be more like the other members of their society. It allowed its people to be more modern and secular in their outlook. This is what distinguishes it as a movement within the Jewish faith.