What is the difference between tags parent, dependency and plugin (Maven) - maven

Question

I am confused by the fact that POM.xml contains parent, dependency and plugin. Somethings something specified with parent must be again included in dependency, for example
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.mycompany.app</groupId>
<artifactId>my-app</artifactId>
<packaging>jar</packaging>
<version>1.0-SNAPSHOT</version>
<name>my-app</name>
<url>http://maven.apache.org</url>
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>1.2.3.RELEASE</version>
</parent>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>3.8.1</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context</artifactId>
<version>4.1.6.RELEASE</version>
</dependency>
</dependencies>
</project>
Why do I have to specify a dependency twice ? How can I use these three tags ?

Answer 1

The parent is a POM which gets "inherited", it basically gets combined with your current POM by means of overwriting. (You can use "mvn help:effective-pom" to print the combined result). So all the dependencies and plugins in the parent will become yours (and all managed dependency versions, which is in case of spring-boot a majority).
The dependendency specifies artifacts you need on your classpath for compiling or testing or running your project (depending on its scope).
Whereas plugin is a artifact which gets executed at build time (like a compiler, report generator and stuff).
I would suggest to read a good tutorial or book on maven, my simple explanation is not enough to make good use of it. https://maven.apache.org/articles.html