A "web application" is a loosely defined term that refers to "applications" that are intended to be used in the web environment. This commonly, but perhaps not necessarily, means that they will be running inside a web browser.
"HTML tags" are definitions enclosed in HTML documents inside a pair of angle brackets ( < > ), and are used to describe the flow of the structure of the document, "marking up" elements inside and around them with special meanings.
Responsive Web Design
Responsive web design is not a technology, product or a programming language. Rather, it is an approach to the design and development of web content wherein through the use of certain modern web techniques, the designs and layouts of a website or web applications are made with the ability to respond to the changes in screen size, and to layout themselves accordingly.
Media queries are conditional statements written for CSS that allow web developers to write layout rules that are only applied when certain criteria related to the screen or display environment are met. Commonly, this is used to enable different layouts depending on the screen or widow size.
CSS (Cascading Style Sheets) is a web development language used to describe the presentation of an HTML document when displayed or rendered
Selecting specific elements of web pages using CSS
HTML5 is a very popular markup language used to describe hypertext documents that are commonly used to define the structure and content of web pages on the Internet. More broadly speaking, HTML5 can also been seen to cover a larger set of technologies and languages that are used to provide not just the structure, but also the functionality, look and feel, and other features and aspects of web applications.
Reverse Domain Notation
The "reverse domain notation" is a common way to generate unique identifiers that are based on Internet domain names. As the name suggests, an identifier expressed in reverse domain notation is the same as the actual Internet domain name, but expressed in reverse.