Data visualization is one of the most important steps to consider during any GIS project. The way in which your data is visualized will directly impact your audience's interpretation of the final product. Visualization elements include coloring, map extent, labels, boundaries, interactivity, 3D models, and more.
See Esri's blog post, Design principles for cartography for an introduction to using colors, text, legends, and other cartographic elements in your maps.
Image retrieved from: http://www.esri.com/software/arcgis/arcgisonline/features/visualization
Below are some common map elements and design attributes to consider when visualizing your data: