Ecumenism is the movement within the various Christian bodies to work together on common causes and re-assess those beliefs and peculiarities that separate us.
Examples of Protestant ecumenism are the National and World Councils of Churches. Many individual Protestant bodies have worked ecumenically to bring harmony to their beliefs so as to allow for common communion, and/or interchangeability of clergy, ie reaching Full Communion, where a member of one church can go and participate fully in a church of the other body. I believe that the Episcopalian Church and the ELCA have such an arrangement.
The Catholic Church approaches ecumenism from a different angle. Inter-communion is not possible with churches lacking a valid Apostolic priesthood and proper Eucharist, but the Church endeavors to work with other Christian bodies in areas of agreement (moral issues like abortion) and to engage in dialogues with groups like the Lutherans and Anglicans about divisive issues, trying to reach common understandings.
A fruit of this is the decree on common understanding of justification published by the Catholic Church and the Lutheran World Federation (which includes the ELCA), effectively harmonizing Catholic and classical Lutheran soteriology.
Perhaps the focus of Catholic ecumenism is the Eastern Church, though unfortunately, there is not as much of a response from those bodies, who have suffered 100 years of Soviet persecution and are just now getting back on their feet, and thus are not in a position to be engaging in dialogues aimed towards resolving the millenial schism. Unlike talks with, say, the Southern Baptist Convention, talks with the Orthodox focus on only one thing: reunion and what it will take. Since Rome accepts the validity of Orthodox priesthood and sacraments, the barriers towards more unity between what John Paul II calls "the two lungs of Christ's Church" is far more reasonable than any hope of major agreement between Catholicism and some Protestant groups, especially those way off in the left field of fundamentalism.
Kirk