The Built-In Way
If you just need a quick capture, Mac already has this baked in. Command-Shift-3 gives you the whole screen. Command-Shift-4 turns your cursor into a crosshair so you can drag and pick a region. Command-Shift-4 then Space lets you click on a specific window and grab just that thing with its drop shadow intact. I've been doing this for years and honestly these three shortcuts cover maybe 90 percent of what I actually need. Command-Shift-3, Command-Shift-4, Command-Shift-4 then Space. That's the short answer to the question. The files show up on your desktop by default, labeled Screen Shot followed by a date and time stamp. You can change where they save if you want by holding Control while hitting one of those combos, which puts the image on your clipboard instead so you can paste it directly somewhere. I should mention one thing people run into constantly. If you press Command-Shift-4 and your cursor suddenly turns into a camera icon but nothing happens when you click, check whether you have a third-party tool like CleanShot or Lightshot running in the background. Those apps often intercept the keyboard shortcuts before macOS gets to them. I wasted about ten minutes once thinking my keyboard was broken because CleanShot was grabbing Command-Shift-4 and I had forgotten it was active. Uninstalling or quitting the app fixed it immediately.
The Screenshot App
Starting with macOS Mojave, Apple added a more visual interface for this. Command-Shift-5 opens a toolbar at the bottom of the screen with options for capturing the whole display, a selected window, or a selected portion. It also has a timer option and a button to record video of your screen. This is useful when you need a delay between when you trigger the capture and when it actually happens, which comes in handy for taking menus or tooltips that disappear on interaction. The downside people don't talk about enough is that the toolbar itself gets included in any full-screen capture if you're not paying attention. Command-Shift-5 captures the whole screen, and if you don't switch to a different capture mode, you'll get the toolbar in your screenshot. Just an annoying thing to deal with when you're trying to document something clean.
Third-Party Tools
There are dedicated screenshot apps that do things the built-in tools can't, like scrolling captures. macOS doesn't have native support for scrolling screenshots of a webpage or a long document. CleanShot X handles this, as does Shottr. If you work with a lot of UI documentation or web content, these tools usually save you from having to stitch multiple images together manually. They also tend to have annotation tools that are better than what Apple gives you, with arrow and blur features that are actually usable for creating labeled screenshots for tickets or guides. The tradeoff is cost. Most of these are paid apps, generally in the ten to thirty dollar range. The free alternatives are limited. I tried using some of the freemium options before settling on what I use now, and the free tiers tend to hit you with watermarks or download limits that make them impractical for regular use.
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Edge Cases and Workarounds
Here's a scenario that trips people up. If you're on a MacBook with Touch Bar, Command-Shift-3 still works for full-screen captures, but Command-Shift-4 sometimes behaves differently depending on your macOS version and whether you're using external monitors. On my machine, which runs a dual-monitor setup with a MacBook Pro, Command-Shift-4 on the secondary display occasionally captures the wrong screen if your cursor is positioned between displays when you press the shortcut. The workaround is to make sure your cursor is fully over the display you want to capture before pressing Command-Shift-4. It's a minor habit to develop, but it prevents a lot of frustration. Another thing worth knowing: if you hold Option while dragging to select a region with Command-Shift-4, the selection starts from the center instead of the corner. This makes it easier to roughly center your capture area without having to fiddle with the starting point. It's not intuitive at all, but it's there if you need it. The built-in screenshot functionality on Mac is straightforward for most use cases, but if you need anything beyond basic captures, you'll likely find the limitations quickly. Scrolling captures, delayed captures on specific windows, and batch operations aren't well-supported out of the box. That's when deciding to invest in a third-party tool becomes less optional and more practical.