A front-end developer is a type of software developer who specializes in creating and designing the user interface (UI) and user experience (UX) of websites and web applications. The primary responsibility of a front-end developer is to ensure that the visual and interactive aspects of a website or application are user-friendly, aesthetically pleasing, and functionally efficient.

HTML stands for HyperText Markup Language. It is a markup language used to create web pages and applications that can be displayed on the internet. HTML is a straightforward computer coding language that was developed in the 90s. It is used to design web pages using a markup language. HTML is the combination of Hypertext and Markup language. Hypertext defines the link between web pages. HTML tags are used to define HTML elements. An HTML element usually consists of a start tag and an end tag, with the content inserted in between. HTML tags are used to create HTML documents and render their content on web browsers. HTML determines the structure of web pages.

Cascading Style Sheets (CSS) is a stylesheet language used to describe the presentation of a document written in HTML or XML (including XML dialects such as SVG, MathML or XHTML). CSS describes how elements should be rendered on screen, on paper, in speech, or on other media. CSS is a style sheet or presentational language that is used to layout, format, and style documents that are written in HTML to make them look beautiful. CSS is generally used with HTML to change the style of web pages and user interfaces. Basically CSS gives the outer cover on any HTML elements. If you consider HTML as a skeleton of the web-page then the CSS will be the skin of the skeleton.

Bootstrap is a front-end open source framework for web application design which consists of HTML, CSS and JavaScript for developing responsive, mobile-first website. Bootstrap, originally named Twitter Blueprint framework and was developed by Mark Otto and Jacob Thornton at Twitter. Bootstrap is completely free to download and use!
It maintains UI consistency, encourage consistency across internal tools. By using Bootstrap, you can create attractive web components like alert, tabs, carousel, and menu etc using CSS and JavaScript. It facilitates users to develop a responsive website. It is compatible on most of browsers like Chrome, Firefox, Internet Explorer, Safari and Opera etc.Bootstrap is a free collection of tools for creating a websites and web applications.

JavaScript is a weakly typed language (dynamically typed). JavaScript can be used for Client-side developments as well as Server-side developments. JavaScript is both an imperative and declarative type of language. JavaScript contains a standard library of objects, like Array, Date, and Math, and a core set of language elements like operators, control structures, and statements.
JavaScript is a lightweight, cross-platform, single-threaded, and interpreted compiled programming language. It is also known as the scripting language for webpages. It is well-known for the development of web pages, and many non-browser environments also use it. With JavaScript, users can build modern web applications to interact directly without reloading the page every time.