In the context of Salesforce development, Apex classes and Apex triggers serve different purposes and have distinct characteristics:

Apex Classes:
Definition: Apex classes are similar to classes in other object-oriented programming languages. They are used to define data and behavior within Salesforce.
Purpose:
Encapsulation: They encapsulate logic that can be reused across different contexts or transactions.
Custom Functionality: They allow developers to implement custom business logic, create web services, and manage complex data operations.
Organization: Classes can organize code into logical units, making maintenance and understanding of the codebase simpler.
Usage:
Can be instantiated or used as static classes.
Can contain methods, properties, and inner classes.
Can be called from triggers, other classes, Visualforce pages, or through API calls.
Example:
apex
public class AccountManager {
public static void updateAccountRating(List<Account> accounts) {
for (Account acc : accounts) {
if (acc.AnnualRevenue > 1000000) {
acc.Rating = 'Hot';
}
}
}
}

Apex Triggers:
Definition: Triggers are pieces of Apex code that execute before or after specific data manipulation language (DML) events occur, like insert, update, or delete of records.
Purpose:
Automation: They automate processes or enforce rules when records are created, edited, or deleted.
Data Integrity: Ensure data consistency and integrity by running checks or validations before data is committed to the database.
Usage:
Triggers fire automatically based on the event type (before/after insert, update, delete, undelete).
Limited to one trigger per sObject (e.g., Account, Contact) per event type (before/after), but can contain multiple operations through conditional logic.
Can call methods in Apex classes to perform logic.
Example:
apex
trigger AccountRatingTrigger on Account (before insert, before update) {
AccountManager.updateAccountRating(Trigger.new);
}

Key Differences:
Invocation: Classes are manually invoked, whereas triggers automatically respond to database events.
Scope: Classes can be broader in scope, dealing with any aspect of business logic, while triggers are specifically tied to database operations on records.
Flexibility: Classes offer more flexibility in terms of structure and usage; triggers are constrained by when and how they can be executed.
Reusability: Code in classes can be reused across different parts of an application, making them very modular. Triggers, while they can call methods in classes, are less modular due to their event-driven nature.

Understanding these differences is crucial for designing effective Salesforce applications where both classes and triggers play integral roles in handling business logic and data management.