Simple and fresh are the main design trends today, oftern reffered to as 'flat' design and 'material' design which have taken over from the outdated 'skeuomorphic' design concept.
Flat design is clear and simple, using colour and simple icons to guide the user on the screen.
Microsoft and Google pushed flat design and the clean unfussy look soon gained popularity.
Material design takes the clean flat design concept a step further by adding a thin 3D effect on the screen with subtle animation.
In this example you can see the panel on the left floating on the background map by adding a slight shadow it mimics a sheet of paper resting on the map just like the 'material' world.
How it used to be
Skeuomorphic design makes use of realistic objects or textures and uses them on screen. This concept was made famous by Apple on their iMac which had the glossy plastic effect skin on their OS and website.
Apple continued to use skeuomorphic design until a few years ago.