Timelines are a feature of task lists in SharePoint 2013/Online. They’re unexpectedly cool, really, you can color code them, drag events around (see below, just click and drag events on the timeline to move them)… they look pretty neat.
They make great project schedule displays for milestone tracking! Items with just an end date display as a pin on the timeline, those with both a start and end date display as a bar. You can add or remove items from the timeline by clicking the “…” next to an item and then “add to timeline”. You can also choose the color of the items on your timeline by clicking them (in the timeline, not the task list) and using the editor in the ribbon!
You can display them from task lists on other sites!
Timelines are one of the few things that can be displayed on sites other than the one they live on in SharePoint using the Timeline web part. Just give it the URL of your task list in the web part settings to pull one into another site. You can also use this to display JUST the timeline (not the task list) on a page in the same site.