The front-end of a web app is the part a user sees and directly interacts with it. It is built using CSS, HTML, and JavaScript. Front-end web app developers use CSS, HTML, and JavaScript to code up web apps.
Some web apps are only built with CSS, HTML, and JavaScript. However, other web apps have more source code hidden away in the backend of web apps to enhance or augment the front-end of web apps.
Back-end web app developers generally work with a front-end web developer to make their code work within the website’s design (or to tweak that design whenever necessary) and front-end.
On the other full-stack web developers and engineers work with both the front-end and backend of a website. They are familiar with HTML, CSS, JavaScript, and one or more backend programming languages.
Most full-stack engineers or programmers specialize in a particular backend language, like PHP or Ruby, or Python, although some, especially if they have been working as a web app developer for a while, work with more than one.