Maven is a build automation tool. From Selenium perspective we should be mainly understanding the following sections of the POM (Project Object Model)

1. DEPENDENCIES : 
           Selenium Web-driver mainly depends on  jar for compilation and execution of program. Also if we implement Java Framework - TESTNG / JUNIT, we need add the dependencies for TESTNG / JUNIT as well. Also dependencies gives you the information, that what is the version of the jar we are using for compiling and executing the program.
Also, all the required jars are downloaded into the folder c:\\users\.m2 folder.
.m2 folder is repository folder created by maven, used for referring to the jars required for compilation and execution of program.

SELENIUM DEPENDENCIES:
<dependency>
<groupId>org.seleniumhq.selenium</groupId>
<artifactId>selenium-java</artifactId>
<version>2.45.0</version>
</dependency>
REFERENCE:
http://mvnrepository.com/artifact/org.testng/testng/6.8.8

JUNIT DEPENDENCIES:
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.4</version>
</dependency>
REFERENCE:
http://mvnrepository.com/artifact/junit/junit/4.4       
         
2. PLUGINS

Plugins helps to execute the JUNIT/TESTNG programs to execute.
Also in case of JUNIT we have to include Suite class. And in the case of TESTNG we have to include the testng.xml file.

JUNIT PLUGIN INFORMATION
<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-surefire-plugin</artifactId>
    <version>2.18.1</version>
    <dependencies>
      <dependency>
        <groupId>org.apache.maven.surefire</groupId>
        <artifactId>surefire-junit47</artifactId>
        <version>2.18.1</version>
      </dependency>
    </dependencies>
    <configuration>
<includes>
            <include>JunitTestSuite.java</include>
          </includes>
    </configuration>
</plugin>
REFERENCES
http://maven.apache.org/surefire/maven-surefire-plugin/examples/inclusion-exclusion.html

TESTNG PLUGIN INFORMATION:
<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-surefire-plugin</artifactId>
    <version>2.18.1</version>
    <dependencies>
      <dependency>
        <groupId>org.apache.maven.surefire</groupId>
        <artifactId>surefire-junit47</artifactId>
        <version>2.18.1</version>
      </dependency>
    </dependencies>
    <configuration>
<includes>
            <include>testng.xml</include>
          </includes>
    </configuration>

</plugin>
REFERENCES:
http://maven.apache.org/surefire/maven-surefire-plugin/examples/testng.html

Page Object Model is the most popular Design pattern used by the developers as a best practice to develop the project.  Before get into the Page Object Model lets discuss the steps to follow once joined as a Automation Tester in the Company.
Steps to Follow:
1. Acquire Domain knowledge for the project.
2. Understand the (SRS) Software Requirement Specification. 
3. Understanding Manual Test cases Ex: Sanity or Regression Test cases     
4. Executing manual Test cases     
5. Identify the Actions perform in each and every page.  
Example: Consider the website Wikipedia. In the main page user can do the search by providing the input and Select the Language and click on Go button as shown below:








void enterText(String inputData)- To enter the text the Search Text-box Ex: India
void selectLanguage(String lang)- To Select a language Ex: English
void clickOnNext()-> To click on the Go Button
boolean verifyText(String expected)->To verify the Text
SearchResultsPage search(String inputData,String lang)-> Clicking on the Go button, navigates to the Search Result Page. So, this function returns an next page object.
Usually the Methods return the Other Page Objects.
6. Write down the methods in every Page.java. 
7. Identify common functions related to Framework.  
getDriverInstance(String browserType)
launchApp(String url)
closeBrowser()
readExcel(String FileName)     
8. Write testcases using Testng Java Framework and invoke the Page related methods in testcases as shown the below program:
Testng Program: WikiTests.java
package com.wiki.tests;
import org.junit.Assert;
import org.openqa.selenium.WebDriver;
import org.testng.annotations.BeforeClass;
import org.testng.annotations.Parameters;
import org.testng.annotations.Test;
import com.wiki.lib.AppLibrary;
import com.wiki.pages.HomePage;
import com.wiki.pages.SearchResultsPage;

public class WikiTests {

AppLibrary appLib;
static WebDriver driver;
HomePage hPage;
SearchResultsPage sPage;

@Parameters({"browserType","url"})
@BeforeClass
public void invokeBrowser(String browserType,String url)
{
appLib = new AppLibrary();
driver =appLib.getDriverInstance(browserType);
appLib.launchApp(url);

}
@Parameters({"searchInput","lang"})
@Test
public void verifySearchResults(String searchInput,String lang)
{
hPage = new HomePage(driver);
sPage = hPage.search(searchInput,lang);
boolean result = sPage.verifyText(searchInput);
Assert.assertTrue(result);
}
}
HomePage.java
package com.wiki.pages;
import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.support.ui.Select;
public class HomePage {
WebDriver driver;
public HomePage(WebDriver driver) 
{
this.driver= driver;
}
public void enterText(String inputData)
{
                    driver.findElement(By.id("searchInput")).sendKeys(inputData);
}
public void selectLanguage(String lang)
{
Select select = new Select(driver.findElement(By.id("searchLanguage")));
select.selectByVisibleText(lang);
}
public void clickOnNext()
{
driver.findElement(By.name("go")).click();
}
 
public boolean verifyText(String expected)
{
String actual = driver.findElement(By.xpath("//a[@href='//www.wikidata.org/']/text()")).getText();
boolean b=false;
if(actual.equals(expected))
{
b= true;
}
return b;

}
public SearchResultsPage search(String inputData,String lang)
{
enterText(inputData);  
selectLanguage(lang) ;  
clickOnNext();  

SearchResultsPage s1 =new SearchResultsPage(driver);
return s1;
}



}
References:
 https://code.google.com/p/selenium/wiki/PageObjects
 Advantages of Page Object Model: 
1. Maintenance of Code is easily  done.   
2. Duplication of Code is removed   
3. Save Time & Effort   

TestNG is the java framework mainly useful to automate the manual testcases and insert verification points.

1. Testng & Junit are both Java Frameworks used by developers to perform unit testing of their code.
2. Junit is subset of Testng. Testng is so powerful due to its advanced features which are as below

a. Grouping of testcases- Group=Sanity 10testcases,Group=Regression 100-Done
b. Paramterization of testcases
c. Parlell execution of testcass
d. Passing multiple values to the testcases using Excel-Done
3. Testng generates very good html reports which are useful to showcase to the stake holders.

Select the project and Press f5 to the see the test-output folder.
a. index.html
b. emailable-report.html


4. TEstng doesnt have main method
5. Testng has annotations.
6. Testng executes the @Test methods in alphabetical order.

For all the testng documentation pls refer to this link ->http://testng.org/doc/documentation-main.html