A queue is a data structure that operates on the principle of 'First In, First Out' (FIFO). This means that the first element inserted into the queue is the first one to be removed.

Think of it like a real-life queue: those who arrive first are also served first. In computer science and message processing, a queue is used to store elements or messages waiting to be processed by a process, application, or system.

For instance, a message queue in a message broker works similarly. When an application sends a message, it's placed in the queue, waiting there until it's picked up and processed by another application or system. This facilitates efficient, ordered, and timed processing of messages or tasks.

A Message Broker is a software component that facilitates communication between different applications or systems by receiving, forwarding, and delivering messages. It acts as an intermediary, transporting messages from one application to another regardless of the type of application or its location.

The Message Broker receives messages from a sending application, temporarily stores them, and then forwards them to the respective receivers. The broker can provide various functions such as message queues, topics, message routing, and transformations to ensure that messages are transmitted efficiently and securely.

Such systems are often used in distributed application landscapes to facilitate interaction and data exchange between different applications, services, or systems by enabling loosely coupled, reliable communication.

A web application is a software application accessible via a web browser and operates over the internet. Unlike traditional software installed on a local computer, a web application runs on a remote server and is accessed through the user's browser.

Web applications can encompass a wide range of functions, from simple interactive pages to complex applications such as social networks, email services, online stores, productivity tools, and more. They often use a combination of different technologies like HTML, CSS, and JavaScript on the client-side (in the user's browser) as well as backend technologies like databases, server-side scripting languages (e.g., Python, PHP, Ruby), and frameworks to support functionality.

Accessing web applications via the browser makes them platform-independent, allowing them to be used from various devices with an internet connection—be it a computer, tablet, or smartphone.