The mekhitza is a divider between men and women in a synagogue. Sometimes it is simply a physical separation between the two sexes, such as a room divider or curtain. Sometimes the women's section is in a balcony. The basis for a mekhitza is from the Ezrat Nashim (women's section) part of the Temple.
The reason for a mekhitza is to provide the men with a greater opportunity to focus on their prayers, rather than on the attractive women in the room. A second reason for the separation stems from early religious rituals coinciding with the start of Judaism. These rituals involved sexual activities, and a separation between men and women helped decrease chances of Judaism's religious practices following the customs of their neighbors.
Although the mekhitza does not exist specifically to relegate Jewish women to a lesser status, it inadvertently does so at synagogues where the mekhitza does not provide enough space, or is located in a place where the women feel their presence is an annoyance. Because women do not participate in Orthodox services, they do not always attend. The women's section is stereotypically a place where women talk without regard for the prayer service. This is not always the case, however. Some synagogues have zero tolerance for talking, even in the women's section. In addition, there are many synagogues, where a mekhitza provides women with a chance to hear and see the synagogue service in its entirety.
Sources: Telushkin, Joseph. Jewish Literacy: The Most Important Things to Know About the Jewish Religion, Its People and Its History. NY: William Morrow and Co., 1991 and Judaism 101.