// ABOUTME: Controller extension for Account pages.
// ABOUTME: Adds custom query logic on top of the standard Account controller.
public class AccountExtension {
    private final Account acct;

    public AccountExtension(ApexPages.StandardController stdController) {
        this.acct = (Account) stdController.getRecord();
    }

    public List<Contact> getRelatedContacts() {
        return [SELECT Id, Name, Email FROM Contact WHERE AccountId = :acct.Id];
    }

    public PageReference doCustomSave() {
        update acct;
        return new PageReference('/' + acct.Id);
    }
}

<!-- LEGACY Visualforce page using standard controller + extension -->
<apex:page standardController="Account" extensions="AccountExtension">
    <apex:form>
        <apex:pageBlock title="Account Details">
            <apex:inputField value="{!Account.Name}" />
            <apex:pageBlockTable value="{!relatedContacts}" var="c">
                <apex:column value="{!c.Name}" />
                <apex:column value="{!c.Email}" />
            </apex:pageBlockTable>
            <apex:commandButton value="Save" action="{!doCustomSave}" />
        </apex:pageBlock>
    </apex:form>
</apex:page>

<!-- LEGACY: apex:actionFunction wiring -->
<apex:page controller="MyController">
    <apex:form>
        <apex:actionFunction name="jsRefresh"
                             action="{!refreshData}"
                             reRender="resultsPanel"
                             oncomplete="handleComplete();" />

        <apex:actionFunction name="jsUpdateName"
                             action="{!updateName}"
                             reRender="resultsPanel">
            <apex:param name="newName" assignTo="{!nameParam}" value="" />
        </apex:actionFunction>

        <button type="button" onclick="jsRefresh();">Refresh</button>
        <button type="button" onclick="jsUpdateName('Acme Corp');">Update</button>

        <apex:outputPanel id="resultsPanel">
            {!resultMessage}
        </apex:outputPanel>
    </apex:form>
</apex:page>

// ABOUTME: Controller with RemoteAction for high-performance JS remoting.
// ABOUTME: Returns serialized data directly to client-side JavaScript.
public class RemotingController {
    @RemoteAction
    public static String doSomething(String param) {
        return 'Processed: ' + param;
    }

    @RemoteAction
    public static List<Account> searchAccounts(String term) {
        String wildcard = '%' + String.escapeSingleQuotes(term) + '%';
        return [SELECT Id, Name FROM Account WHERE Name LIKE :wildcard LIMIT 20];
    }
}

<apex:page controller="RemotingController">
    <script>
        function callRemote() {
            Visualforce.remoting.Manager.invokeAction(
                '{!$RemoteAction.RemotingController.doSomething}',
                'hello',
                function(result, event) {
                    if (event.status) {
                        console.log('Result: ' + result);
                    } else {
                        console.error(event.message);
                    }
                },
                { escape: true }
            );
        }
    </script>
    <button onclick="callRemote();">Call Remote</button>
</apex:page>

public class MyController {
    // Included in view state
    public String importantField { get; set; }

    // Excluded from view state — must be re-fetched on each postback
    transient public List<Account> largeResultSet { get; set; }
}