What are Jenkins Shared Libraries and how do you use them?

jenkins-shared-library/
├── vars/
│   ├── buildMaven.groovy     ← Global variable (call as buildMaven())
│   ├── runTests.groovy       ← Call as runTests()
│   └── deployApp.groovy      ← Call as deployApp()
├── src/
│   └── com/company/
│       ├── Utils.groovy      ← Groovy class
│       └── Notifier.groovy   ← Groovy class
└── resources/
    └── templates/
        └── email-template.html

// vars/buildMaven.groovy
def call(String goals = 'clean package') {
    sh "mvn ${goals} -q"
    archiveArtifacts artifacts: 'target/*.jar', fingerprint: true
}

// vars/runTests.groovy
def call(Map config = [:]) {
    def browser = config.browser ?: 'chrome'
    def env     = config.env     ?: 'staging'
    def suite   = config.suite   ?: 'testng.xml'

    sh """
        mvn test \
            -Dbrowser=${browser} \
            -Denv=${env} \
            -Dsurefire.suiteXmlFiles=${suite}
    """

    junit testResults: 'target/surefire-reports/*.xml', allowEmptyResults: true
}

// vars/deployApp.groovy
def call(String environment, String version) {
    withCredentials([string(credentialsId: "deploy-key-${environment}", variable: 'DEPLOY_KEY')]) {
        sh """
            ./scripts/deploy.sh \
                --env=${environment} \
                --version=${version} \
                --key=${DEPLOY_KEY}
        """
    }
    echo "Deployed version ${version} to ${environment}"
}

// vars/sendNotification.groovy
def call(String status) {
    def color = status == 'SUCCESS' ? 'good' : 'danger'
    def msg   = "${status}: ${env.JOB_NAME} #${env.BUILD_NUMBER} - ${env.BUILD_URL}"

    slackSend(
        channel: '#ci-alerts',
        color:   color,
        message: msg,
    )
}

// Jenkinsfile in your project repository

// Import shared library (implicit — if registered as global)
@Library('my-shared-lib') _

// Or specify a version/branch
@Library('my-shared-lib@v2.1') _

pipeline {
    agent any

    stages {
        stage('Build') {
            steps {
                // Call the shared library function
                buildMaven('clean package -DskipTests')
            }
        }

        stage('Test') {
            steps {
                runTests(
                    browser: 'chrome',
                    env:     'staging',
                    suite:   'smoke-testng.xml'
                )
            }
        }

        stage('Deploy') {
            when { branch 'main' }
            steps {
                deployApp('production', '1.5.0')
            }
        }
    }

    post {
        always {
            sendNotification(currentBuild.currentResult)
        }
    }
}