KDE Plasma 5 is a QT based desktop, that has nearly unlimited customization. It's simple to use but also allows you to change anything you can think off in it's extensive collection of settings. Including Themes, workflow. icons, launchers and anything else you can think off.
It's being developed as part of KDE which is one of the biggest open source communities in the world.
KDE also has it's own Wiki for even more info on their apps https://userbase.kde.org/Welcome_to_KDE_UserBase
KDE has a traditional workflow that windows users would be familiar with. With a panel at the bottom a start menu on the left and icons on the right.
helpful widgets you can place on the desktop
activities (separate settings per virtual desktop)
traditional start menu
Quick toggles & clock on the right.
KDE has one of the most extensive settings panels you can find with the ability to change nearly anything. Here's some highlights.
Complete changing of themes, icons, cursors, menus, login screens.
Window management settings.
Full shortcut & custom shortcut editing.
KDE is one of the biggest opensource Eco-systems we will highlights the preinstalled apps but for a full list you can visit https://apps.kde.org/
KDE Connect allows you to sync your android phone with your Linux desktop to get notifications, use it as a mouse. Or to simply send commands to your Desktop!
Discover helps you find and install applications, games, and tools. You can search or browse by category, and look at screenshots and read reviews to help you pick the perfect app.
Dolphin is KDE's file manager that lets you navigate and browse the contents of your hard drives, USB sticks, SD cards, and more. Creating, moving, or deleting files and folders is simple and fast.
KRunner (launched with Alt-F2 or Alt-SPACE) is the launcher built into the Plasma desktop. It can launch programs. switch through tabs, search for folders and files. do calculations and many other things for a full list of features visit https://userbase.kde.org/Plasma/Krunner
Please visit customization page