If the user takes an action where progress can be tracked, the obvious choice is to use a progress bar. File uploads are a good example of when to use progress bars. We already know the file size and can easily show the user how much of the total size has been uploaded. When using progress bars, try not to just show the bar itself. Pair it with text, such as a label describing what the progress bar is for, as well as a numerical representation of the progress (whether it be in units such as bytes/kilobytes/etc or percentages).

If the user takes an action where progress can't be tracked, spinners can be used. That small amount of visual feedback is often enough to keep users engaged and aware of what is happening. You should often add text to go along with your spinners to prompt the user to wait a moment or tell them what to do if the action is taking too long.

Skeleton or ghost states are typically used when a page is in the process of loading. They provide a “wireframe” view of what the user will see once the page is fully loaded. This is particularly good in the case that elements of the page load in at different times. The skeleton loader will keep the page from jumping around as it loads. It is a good idea to use skeleton states even if the page does not take a long time to load. This will greatly benefit your users who are on slower connections.