UI/UX developers are primarily concerned with designing the visual aspect of a digital product, usually using tools like Photoshop, Gimp, Sketch, etc. to create templates/layouts.

More specifically, they are the ones who decide which button, image or text field or forms will be placed on the screen. In general, UI/UX guys don't need to know how to code at all.

On the other hand, a front-end developer is the one who will actually implement these layouts (created by UI/UX developers) using HTML, JavaScript, CSS etc. In this way, they will write the code to place the button or images on the screen.

Note: UI and UX are two different things and although common people use them interchangeably, they have important differences. In terms of career and salary, both fields provide equally lucrative opportunities, but it depends on the skill level an individual possesses.

A front-end developer creates websites using HTML, CSS, JavaScript, and other common client-side technologies. His goal is to deliver a complete product on a confined timeline. A developer sees pages as a whole and a website as a combination of those pages.

A UI-Developer similarly uses HTML, CSS, JavaScript, and other client-side technologies. There is less emphasis on scripting languages like JavaScript and more emphasis on HTML and CSS. His focus is on delivering more granular assets like buttons, tooltips, and other modular pieces that will grow into a larger finished product. He sees an asset as the individual CSS styles and pays close attention to how the asset will look, feel, and behave in different contexts. He may be more familiar with methodologies such as OOCSS, BEM or Smacss. He may not even see a website as a whole, but rather a collection of assets applied to pages.

Basically, "UI developer" and "front-end developer" are just different words for the same thing, because the front-end is the interface of a system. Any small differences would be specific to the culture of a company or person using them.
However, "UI developer" is a less common term. A very popular term from the same family (in case you're looking for it) is user interface designer.

Now, the UI designer is different from the front-end developer (view). As the term suggests, his role is design, not development. His main tools are graphic editors, prototyping software, and a simple pencil.

Front-end developers can also vary in responsibilities. There are front-end developers who draw, design, and code user interfaces, but it doesn't have to be that way. Design and code responsibilities can be divided between UI designers and developers depending on the size of the team, their workload, and talents.

The UI developer works with wireframe graphics - visual assets - UI elements, such as:

- Input Controls: Radio Buttons, Check Boxes, Drop Down Lists, Buttons, List Boxes, Toggles, , Date Field Text Fields

- Navigation Components: Slider, Breadcrumb, Search Field, Pagination, Icons, Labels

- Informational components: icons, tooltips, progress bar, message boxes, notifications, modal windows

On the contrary, the Front End developer does not know much about the design side of the user interface, especially the psychology beyond the user experience (UX). Front End Developer codes what the designer envisioned, the UI developer built and the UX designer approved.

All together he creates a visual side of the front of the site that we all see when we browse the Internet. We see a graphic representation of the informative structure.

Successful front-end developers would know some UI and UX. They see a page as a collection of assets and pay close attention to detail and writing code in a modular way. They constantly analyze how the assets will be used on the page and how they could be used throughout the site.

A website is made up of different layers of data, design, architecture, functionality, and content. Frontend and UI development are two different sides of the same coin and certainly have their own uniqueness. Both frontend and user interface development play a vital role in lucrative website development while the line between the two is often blurred.
In general, these two roles have to work closely in order for a website to be successful.