KDE and GNOME are two of the most popular desktop environments on the Linux operating system. They are developed by two separate communities of developers, with some APIs that are unique and the others, which are common. This makes it easier for third party applications to be developed, as well as there is a common environment to work on. As an end user, you may find each of them to have their own strengths and weaknesses, and depending on which of the two align to your needs more, there can be a significant level of variation in usability.
The KDE and GNOME variants of common distros are often characterized by their naming system itself. For instance, Ubuntu’s default desktop environment is GNOME, while the KDE variant is known as Kubuntu.
For starters, if you look at the plainly superficial and cosmetic differences between the two environments, GNOME is a relatively low frill, dark color favoring desktop environment. On the other hand, KDE has a more aesthetic touch to it, with plenty of sky blues throughout the interface. Moreover, KDE is based on a single button menu style, akin to Microsoft’s Windows. On the other hand, GNOME has multiple buttons for each part of the initial menu.
If you move on to more practical differences, operations like a file renaming process are handled differently in both KDE and GNOME. GNOME handles such processes inline, and does not allow changing of file extensions. In KDE, however, the default option lets you modify the file extension as well.
In case you try to fiddle with any settings, KDE is a rare but safer option. It is an environment with great similarities to Microsoft Windows, in terms of the two-step permission control (that is, saving, and then apply. On the other hand, GNOME simply accepts your choice with one click, and implements it straightaway. Moreover, KDE’s shut down process itself asks you to authorize the terminal, and close it completely in case it is a virtual one. In GNOME, there is a dedicated shutting down button, that on clicking gives you more options like hibernation, restarts, logging off, switching users, etc.
Long story short, KDE focuses on providing its users a highly feature rich interface. Graphical interfaces are used to handle the configuration throughout. Depending on your individual tastes, KDE may come across as highly versatile and powerful, or it may seem to be too careful and convoluted for a beginner to use effectively.
The GNOME environment follows a sense of aesthetics that is nearly Spartan and austere. In the search for usability, many of the lesser used settings are buried deep within menus. Thus, at the cost of an uncluttered user experience, there can be a distinct loss in functionality perceived.
All in all, the two desktop environments provide almost exactly the same functionality; just that the way they go about it can vary greatly. The most interesting part is for the end: in case you still can’t decide which of the two you prefer, you can use them both, and switch when you need to.