Windows 8.1 Update 1 is a set of updates and improvements for Windows 8.1. There are security updates, performance enhancements and other low-level tweaks, but the most obvious changes are to the Windows 8.1 interface.
This starts at boot time, with desktops and laptops now booting to the desktop by default. You can also toggle this setting manually, as before (right-click the taskbar, select Properties > Navigation, select "When I sign in... go to the desktop instead of Start").
Launch the Start Screen and you'll find new "Power" and "Search" buttons, top-right: you can now sleep, shut down or restart your PC in a couple of clicks.
Right-click a Start Screen tile and a context menu appears. Along with the various items which used to appear on the App Bar ("Unpin from Start", "Uninstall", "Resize" and "Turn live tile off"), you can now also pin any app to the taskbar for easier access (the Store comes pinned by default).
The Taskbar includes both Windows 8 apps and desktop programs, and it's even accessible from the Start Screen, at least in theory. Just move your mouse cursor to the bottom of the screen and it should pop up. Unfortunately this didn't always work for us, and we don't know why, but if you're luckier then it'll make it much easier to switch between applications or access pinned shortcuts.
Windows 8 apps have similarly gained a little extra mouse-friendliness. Switch to an app, move your mouse cursor to the top of the screen, and a title bar appears. This has the usual minimise and maximise buttons to the right, while the button to the left also provides "Split Left" and "Split Right" options. The previous Windows 8 mouse actions still work - dragging an app down to close it, for example - but the new options provide a more discoverable alternative.
Please note, you should be able to access Windows 8 Update 1 via Windows Update (just click "Check for updates") but the Download link here will alternatively take you to the official page at Microsoft's Download Center.