My take on it is that a web designer is your Photoshop/Illustrator "Graphic Designer" guy, who loves the ins and outs of CSS and HTML, Layouts, and knows the newest CSS3 features, etc. Sometimes they might go a little into Javascript but usually not to any really deep levels.

The Frontend Developer, in my mind, usually kind of takes off where the "Web Designer" fades out, handling complex Javascript Apps / UI, and AJAX. Besides deep AJAX/JS skills though usually the frontend devs skills start dwindling when it comes to enterprise level PHP / m2m in the DB, etc and thats where your Backend guys come in.

Of course, that's all how I 'imagine' it conceptually, the fact is, unless your working for a large company, I think most web developers do all the above, and call themselves according to whatever area they like the most :)