A snipping tool is a utility that takes a screenshot of your desktop. You can use it to capture the entire screen, specific app windows, or sections that you select with a click-and-drag mouse action.
Windows has two built-in screen-capture options. Press the Print Screen key to simply save a screenshot of the desktop to the clipboard. Or, in Windows, search for the Snipping Tool from your Start menu or taskbar search box. Microsoft's Snipping Tool lets you capture all or part of your screen, save, add notes, or email the clip.
Windows' Snipping Tool may do more than Print Screen, but it's still rather limited. You can't link it directly to the Print Screen key, you can't set the quality of the screenshot, and it has no cloud awareness. Third-party snipping tools can automatically create a screenshot of your screen or part of it, save as a specific file type, and sometimes even upload to a cloud storage account -- all with one or two clicks.
Lightshot is a free tool you can set to pop up whenever you press the print screen key (Shift-Command-4 on Macs), or it can make print screen automatically create an image file. Lightshot will save either a JPEG or a PNG file, and its image quality slider has 100 increments. Unfortunately, you don't get a preview of what the different slider ranges look like, but most tools of this type have no increments to begin with.
Microsoft Snip (Windows only) comes from Microsoft Garage, the company's lab for experimental projects. Handily, Microsoft Snip can be set to start when Windows does, and it has an annotation function.
Greenshot (Windows only) is pretty similar to Lightshot, but it has better cloud support. During installation, you can install plug-ins for Dropbox, Photobucket, Flickr, and others. You can also automate attachment to emails, export to Microsoft Office apps, and highlight or mask parts of the image without having to open an image editor.
While the free apps above only capture screenshots, SnagIt can also capture video, and it comes with more robust tools for editing both photos and videos. SnagIt can also make animated GIFs and record audio, and you get a variety of annotation tools. You can try it for free for 15 days, and if you own an older version, you get a 50% upgrade discount.