Antimonialism is an old word you don't hear very often. Literally, it is "against the law". It is characterized by people who believe they do not need to change their lives as a Christian. These people deny the transforming power of the Holy Spirit and live in sin, contradicting their statements of faith.
Legalism is the opposite extreme. A legalist raises our works (actions) to the point of salvation. In other words, "follow our rules or you're not saved". We are all born under the crushing weight of the Law. A burden which we cannot carry, which ultimately leads to death. When we are saved, we are released from this burden. The legalist returns to this burden.
The Bible makes it clear the proper course is through the middle. Our lives are too complicated to be governed by a set of rules. As Christians, we have freedom. But not everything is good for us. And we must not use our freedom to cause others to stumble or to keep people from hearing the Good News. At the same time, we must place God's will first. We know His will by being steeped in His Word, and applying Biblical principles. We should not be breaking the Ten Commandments. We should be loving others.