Presets are not totally new to 1.8: there already were preset menus on the Doors and Windows property panel and the Create Image tool. Now they have been extended to other tools as well, like Texts, Dimensions, Hatches & Fills, Walls, Columns.
A preset is a set of graphical settings stored on the user's disk. Saving a preset creates a new file that stores all the attributes currently set in the tool panel. When you load a preset, its values are applied to the current tool. Because they are specific to the tool panel, selected objects are not affected by presets until you push the Apply button.
Preset files are stored in the Presets folder in Application Support/ilexsoft/HighDesign/, and each tool creates and lists the content of its own Presets folder. In the Finder, you can organize that tool's Preset folder into subfolders.
Styles are project items with their own properties and behavior, like Layers, Sheets, Views. Drawing objects in HD 1.8 have a new 'style' property that allow you to change all of its graphic properties with a double-click on the Styles palette. You can select all the objects that use a style, and if you modify a style by changing one of its properties and clicking the Update button on the Styles window, all the objects in the drawing that use that style are updated automatically to match the new properties.
When you create a new style, it reads the current properties as set in the Properties bar and, optionally, in the current tool's properties.
A style can be generic or tool-specific. Generic styles store the current properties from the Properties bar, like Layer, stroke color, fill, arrowheads, line type and pen weight. Tool-specific styles, in addition to common properties like Layer and Stroke color, also include all the current properties specific to that tool, if available. When selected, tool-specific styles activate the corresponding drawing tool automatically.
For example, to create a 'Leaders' style, you have to activate the Dimensions tool, Radial/Leaders method, set the desired properties in the Properties Panel, click the "New Style" button on the palette, insert the name, choose the "Tool-specific" option and click any of the common properties available in the table. Now, when you select the 'Leaders' style on the palette, the tool switches to Dimensions - Radial/Leaders and the default properties are changed to those stored in the Style.
Styles are saved into the document, but you can also save a style onto the disk, so you can create you own library of styles to add to new projects as needed.