An agreement is made when two parties agree to something. So, for example, a mother might make an agreement with her son not to kiss him in public because, after kindergarten, well, that's just not cool.
If people's opinions are in accord, or match one another, then they are in agreement. If a teacher assigns a ten-page paper due the next day, the students would be in agreement that the assignment was unfair. A written or verbal contract can also be called an agreement. An agreement to deliver an article a month for a publication could not be broken simply because you have writer's block.