So, if we've got back end developers and front end developers, who makes the website actually look pretty? Who pulls together the images and logos and color schemes? Who worries about whether the user actually focuses on the elements of the page that we want them to? That's typically the web designer's job.
Designers inhabit the most creative side of the web application creation process. Their backgrounds are often in graphic design or illustration or usability. A good web designer thinks from the perspective of the user and focuses on how to make the site as usable as possible.
That means they need to think about both broader issues like how to lay out all of the site's elements on the page and specific things like which icon to use for the "Contact Us" button. They are responsible for delivering the right emotional experience and brand identity for users of the page.
While they have historically been less inclined to handle the actual coding of the page, these days even designers are expected to roll up their sleeves and code a bit, crossing the line into the Front End. Some of the best web developers come from design backgrounds.
And many people choose to play in both realms -- you can absolutely be both a web designer and web developer at the same time. In fact, many freelancers are by necessity so they can keep their cost down and marketabilty up.
So, to summarize, designers tend to worry specifically about the visual and artistic composition of the sites while web developers tend to worry about how to actually build them, but the lines are blurred more and more these days so don't feel boxed in by either description. The best way to learn is to get familiar with the high level of each and then deep dive into whichever one you feel most strongly about.