The Equal Rights Amendment (ERA) was an attempt to add an amendment to the US Constitution that would expressly disallow discrimination based on gender. There have been attempts since 1921 to establish it - the most recent (and most famous) being in the late 1970s to early 1980s, when the amendment actually got to the state conventions for ratification. All attempts have failed, either stalling out in getting the amendment to the table or failing to get the support of the states.
The ERA sounds good, in theory - after all, sex discrimination is bad. That is, until you realize that we already have an amendment that guarantees equal protection to all citizens. Considering that, one has to wonder why the ERA would be brought forth in the first place, since existing law could be used to protect the equal rights of women. There are a few arguments for a rationale coming from both sides - some feel that the equal protection clause doesn't go far enough, while others see the ERA as a sort of end run around the same clause to allow preferential treatment of women. (Personally, I think that it was laziness that motivated it - people didn't want to make the gruling arguments that would connect the equal protection clause to sex discrimination.)
The defeat of the ERA in 1982 was a marked blow for the feminist movement, which had thrown its weight completely behind it. An argument can be made that the radical feminists gained power at this point, much the same way the radical right-wing took control of the Republican Party after Watergate.
For those interested, here is the amendment, in all its glory:
Section 1: Equality of rights under the law shall not be denied or abridged by the United States or by any State on account of sex.
Section 2: The Congress shall have the power to enforce, by appropriate legislation, the provisions of this article.
Section 3: This amendment shall take effect two years after the date of ratification.