Once the people settle in Israel, they are governed by a series of judges, hence the name for this book. These people are not judges in the sense that we would understand today, but instead they should be thought of as informal military heroes who were called and empowered by God to rescue his people from the oppression of surrounding nations.
A reoccurring pattern in the book of Judges is that the Israelites are oppressed, they call out to God for deliverance, he raises up a judge to save them, they turn to God for a time, but later fall away. The cycle repeats.
The actions of some of the judges are noteworthy and significant. Other judges are scarcely mentioned, some only garnering the briefest of citations in a single paragraph or verse.