Shortcut: cmd + G
If you work with layers, you'll often want to apply options to several of them, for example change the color, resize them altogether – or simply just organize things. To be able to do that that you can make a layer group. Just select all the layers you want to group and press cmd + G. This way the layers also keep their aspect ratio if you resize them.
Layer groups are depicted with a folder symbol in the layer list and the nested layers below. To ungroup layers again press shift + cmd + G.
There are also icons for these two options in the toolbar: