gpgemini is incorrect. In Judaism, if your mother was Jewish, you are Jewish. If a man is not circumcised, he is still a Jew.
Anyone over the age of thirteen (12 for a girl, theoretically) who was not born Jewish (i.e. to a Jewish mother) may become Jewish if they accept all of the commandments and submerge themselves in a mikva (ritual bath). For a male to convert, he must also be circumcised, and if he was already circumcised, a small amount of blood must be drawn (from there) to satisfy the requirement.
Since conversion requires full acceptance of all commandments, it must be the person's choice. (Otherwise the witnesses cannot believe the person's statement that they accept in their heart all the commandments)
In fact, Judaism requires converts to be "pushed away" three times to give them every possible chance to back out of the process.
Please Note: All of this applies to Torah Judaism. The Reform and Reconstructionist movements allow someone to be considered a Jew if only the father is Jewish.