Slf4j-log4j12与 log4j-over-slf4j 的区别

Slf4j-log4j12和 Log4j-over-slf4j的区别是什么? 应该在什么时候使用它们？

<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-log4j12</artifactId>
<version>1.7.12</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>log4j-over-slf4j</artifactId>
<version>1.7.12</version>
</dependency>