Take screenshots in the iOS simulator

It's just as simple as command+s or File > Save Screen Shot in iOS Simulator. It will appear on your desktop by default.

An update with Xcode 11.4 simulator

To capture a screen shot, click on 'Camera' icon/action button, on the top bar of simulator.

OR

Select Save Screen Shot menu item, from File menu.

File => Save Screen Shot

Use ⌘ + S to capture a screen shot.
(Use window + s, if you are using windows/non-apple keyboard).

See for more details: How to change simulator scale options with Xcode 9+.

Tip 1: How do you get screen shot with 100% (a scale with actual device size) that can be uploaded on AppStore?

Disable Optimize Rendering for Window scale from Debug menu, before you take a screen shot.

Here is an option

Menubar ▶ Debug ▶ Disable "Optimize Rendering for Window scale"

Tip 2: Where is screen shot saved (Default Path)? How to change default path?

Simulator saves screen shot file on (logged-in user's) desktop and it's default path.

To change default path (with Xcode 9+), press and hold ⌥ Option (alt) button from keyboard, while you take a screen shot.
Or
Using Mac Keyboad: Press keys ⌥ Option + ⌘ + s all together.
It will show to a dialog with file storage options and also allows to set/change default path.

Tip 3: How to take a screen shot with Device Bezel?

Enable Show Device Bezel from Window menu.

Here is an option

Menubar ▶ Window ▶ Enable "Show Device Bezel"

Now,

1. Press ⌘ + shift + 4 + Spacebar, all together in Mac Keyboard. (A window snap capture mode will become active)
2. Select Simulator window/frame, that you want to capture. It will save screenshot with device bezel (with shadow effect in background) on (logged-in user's) desktop.

You can also remove the Simulator toolbar as described here.

on iOS Simulator,

Press Command + control + c or from menu : Edit>Copy Screen

open "Preview" app, Press Command + n or from menu : File> New from clipboard, then you can save command+s

For Retina, activate iOS Simulator then on menu:HardWare>Device>iPhone (Retina) and follow above process

Command + S

is the way to save on Desktop, (on new iPhone simulators, this was introduced in later simulator)