How to edit the frontend of PrestaShop using modules
All the blocks displayed on the frontend of your store are actually modules which you can manage from the backend. So you can remove or add frontend blocks by respectively uninstalling (or disabling) or installing the modules corresponding to those blocks. In this way you can use the modules to make different changes to the frontend of the store and alter its appearance.
Different themes are organized in different ways. For example, the default theme that comes with the PrestaShop installation package has a header and a footer, a left and right column, and a middle section. So, for example, you can remove blocks from these sections by uninstalling/disabling the respective modules.
Most of the modules that appear on the frontend can be found in the Front Office Features section of the backend's Modules tab. You can uninstall or disable the particular module by clicking on the respective button for that module. In the same way you can also install modules that are uploaded to the modules folder on your hosting account, but are uninstalled.
Some modules also have a Configure button which you can use to edit the settings for that module. A good example is the module Home text editor. From the settings of that module you can change the introductory text and the image logo that are displayed in the middle section of your store's homepage.
You can also move the positions of some of the modules displayed on the frontend. This can be done from the Positions sub-tab of the backend's Modules tab.
For some additional information and screenshots check out the tutorial on editing the frontend of your PrestaShop by managing modules.