Device type handler, device handler, or device type. It’s a small bit of code that tells SmartThings how to communicate with a specific type of connected hardware. Officially integrated devices all use stock DTH’s that are built into the ST platform. Some devices require installing a custom DTH to work with SmartThings, or to expose functionality missing from the stock DTH.
More info can be found in this FAQ:
https://community.smartthings.com/t/faq-an-overview-of-using-custom-code-in-smartthings-smartthings-classic/16772
ActionTiles is a customizable dashboard program that allows you to view and control your SmartThings devices from any browser. There are several of these dashboard programs available, ActionTiles is just one of the more popular ones.
More info on ActionTiles can be found here: https://community.smartthings.com/t/release-v6-8-x-actiontiles-formerly-smarttiles-v6-theme-editor-builder-added/126686
You can also search the forum for “dashboard” to see some other dashboard options and ideas.
Edit: I was way too slow in my response, others already have it covered