Let’s say a friend sends you a link to an app or an MP3 file by email. Downloading the thing usually means copying the URL, opening your browser, pasting in the URL, and then selecting File > Save As to save the file. To save yourself a few steps, you can create a service in Automator that will do the downloading for you.
Open Automator (found in Applications > Automator, or just use the Spotlight search function (click on the magnifying glass at the very top right of your desktop) and select the Service template. At the top right of the window, set the service to receive selected URLs in any application. Then, in the Internet group of actions, double-click on Download URLs.
By default, that action will download URLs to your Downloads folder; if you wish them to go somewhere else, select that other location from the Where drop-down menu. Save the service and give it a name, such as ‘Download Selected URL’.
Now, in any app that supports services, find the URL of a downloadable file and select it with your cursor. Open the Services submenu from the application menu, and select Download Selected URL from the list. (Or Control-click on the selected URL and choose the service from the contextual menu’s Services section.) The file connected to that URL should download to your specified folder.