I have Responsive Layout Maker but haven't had a chance to use it much.

I was also looking at Responsive Web Designer, but I'm confused as to what is so different about it.

Couldn't you use either to make a fully responsive website?

They both can make responsive sites. I have both and what I can say is it depends on how you work and your ability.
For me I bought RLM but was unable to really get anything from it because of how I work. I bought RSD when it came out and I am making progress with that. I need, at least to start with, pre-made templates that I can adapt to my needs until I get a feel for things. I am currently redoing my personal site and find some success with the pre-made template. Right now my site is a three way mix of RSD, VSD and a responsive photo gallery that I got from somewhere, can't remember where, when CC did not have a responsive gallery of their own.

AngelArs, if you are making just a site or two for yourself and friends, I think RSD will suit your needs. I think people find it easier to learn and get work done with it, especially if they are not used to writing code.
If, however, you are makig sites for paying customers, you may want RLM(P) in order to quickly make a mock-up design to show them. You can then import your .rlm file into RSD for further developing and adding text, images etc.

Both can be used to create a responsive website.

Responsive Layout Maker is great if you're the kind of designer who likes to get "under the hood" and work with code. RLM solves one of the biggest challenges in responsive design, having a flexible core which can adapt based on the viewer’s screen size. In a nutshell, Responsive Layout Maker creates a responsive prototype website (called a layout). You build the structure of your website in Responsive Layout Maker and then export to any code editor to add your final content and design styles.

Responsive Site Designer on the other hand, allows you to add the design layer visually without having to dive into the code. These design tools allow you to add background images, gradients, shadow, radius, and set state functionality (hover, active, visited). There's more cool stuff like a web inspector tool, adding meta data and footer code, integrating and customizing font icons (1200+ icons come with the app) and more.

Get more details on the comparison here: http://www.coffeecup.com/help/articles/1423

OK thanks everyone. From the looks of it I think I bought the wrong one. I probably should have bought Responsive Web Designer instead of Responsive Layout Maker.