what is the difference between a usb- and a lightning connection?

A Universal Serial Bus (USB) is a single plug that combines power delivery and data in a single plug.

There have been various advancements over the years. The current standard that Apple has adopted is USB Type-C because it has much faster data transfer speed, a convenience of daisy-chaining other devices, a reversible plug, compatibility with Thunderbolt connectors and has a much smaller plug size for portability.

The lightning connection is an Apple innovation that offers a dedicated standard that meets the higher than average quality, is a much smaller size and offers a reversible plug. 

Lightening plugs are used on iPhones and iPads for charging and accessories. The iMac doesn’t have a Lightening port but latest iMacs do come with a USB-C to Lightening cable for charging the Magic Mouse & Keyboard, and for connecting an iPhone to the iMac.