parent-pom.xml

<dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8</version>
    </dependency>
 </dependencies>
</dependencyManagement>

child-pom.xml

<dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
    </dependency>
 </dependencies>

Two Main Differeneces

1. Dependency Management allows to consolidate and centralize the management of dependency versions without adding dependencies which are inherited by all children. This is especially useful when you have a set of projects (i.e. more than one) that inherits a common parent.Another extremely important use case of dependencyManagement is the control of versions of artifacts used in transitive dependencies.You still need to define them in the child POM to show that you are using them. They are not actually included in the child projects just because they are in in the parent POM. Enclosing dependencies in centralizes management of the version, scope, and exclusions for each dependency, if and when you decide to use it.
2. Artifacts specified in the section <dependencies> will ALWAYS be included as a dependency of the child module(s).Artifacts specified in the section will only be included in the child module if they were also specified in the section of the child module itself. Why is it good you ask? because you specify the version and/or scope in the parent, and you can leave them out when specifying the dependencies in the child POM. This can help you use unified versions for dependencies for child modules, without specifying the version in each child module.

To sum up the above points in simple words in the parent-pom.xml if you are adding things inside tag then it would be available to the child-pom.xml. If you are adding the same thing within tag then you should manually add them to the child-pom.xml.In the child-pom.xml if the version is not specified it will take the parent-pom.xml version of the dependency as default else you can override them by specifying one in child-pom.xml

References Read It