The Layer List contains a list of all layers (including Slices and Artboards) on the current page and shows a small preview for each. You can also see which one is locked, invisible, has a mask, or is marked for export. Here you can reorder layers and choose Boolean Operations for subpaths of a shape. You can also group layers or rename them to keep your document organised.
Sketch does have support for multiple pages and you can switch to other pages using the little popup above the Layer List (you can also use the Page Up / Page Down keys on your keyboard). The Layer List always shows the layers of the current page only.
If you want to add/remove pages, or drop layers between pages, you can open the page list widget:
Click the + icon to add a page. Right click a page to get a menu to delete or duplicate a page:
You can reorder pages by dragging and dropping.
Also, you can drag layers and Artboards from one page to another (hold ⌥ to duplicate them):
Artboards are displayed with a white background in the Layer List. You can consider them the top-level objects in your design and as such one Artboard cannot be inserted into another Artboard.
Layers that have a mask applied to them will show a little arrow in the Layer List before their name and preview. The layer that is casting the mask is the first layer below it without a dot. More about masks.
Shapes can contain multiple subpaths. If so, they’re displayed as a group in the Layer List, with a disclosure arrow to show the sub-paths. Each sub-path can have a boolean operation applied to it which dictates how it is combined with the path below it, which is then fed to the path above it. The Layer List is a convenient place to get an overview of how the subpaths work together to achieve the final result and they’re an easy place to change the operations if you want to do so. More about Boolean Operations.
Symbols and Shared Styles
Symbols are a special kind of group, which can exist in your document in multiple places. They’re easily identified by the purple folder icon the Layer List uses to display them - as opposed to a blue one for normal folders. More about Symbols.
Shared Styles are used to keep styling consistent between multiple objects and they can be applied to shapes and text layers. To indicate that a shape or text layer is using a shared style, their preview is displayed in a purple color instead of the standard dark shade of grey. More about shared styles.
Hiding and Locking Layers
Any layer or group can be hidden from the Canvas, simply by clicking the eye icon that appears—on hover—to the right of the layer name in the Layer List. If this eye icon is visible, this indicates that the layer is hidden.
Similarly, when holding down the Option key and hovering the Layer List, a padlock icon will appear instead of the hide icon. When clicked, this will show that the layer is now locked on the Canvas, meaning it cannot be selected or moved without being unlocked first.