Wireframes help you focus on the layout aspect of your site. How each element relate to the others, how much white space you should use, measurements etc. Ideally you will have all your content handy in order to really see how it fits.

Mock-ups are the next step. Once you've figured out your layout, it's time to actually design it and make it shine. All the bells and whistles are added there. Make sure you organize your .psd file, as it will help later in the slicing and coding process.