What is Browserstack, Saucelab, Simulator & Emulator?

• BrowserStack

World's leading cross browser testing tool, Web app testing
Interactive web-based testing on 2000+ browsers , operational system and real devices instantly. Say goodbye to your lab of devices and virtual machines.

• Saucelab

Cloud based testing platform, Sauce Labs ensures your favorite mobile apps and websites work flawlessly on every browser, operating system, and device.

• Emulators

Emulators mimic your target device’s hardware and software on your workstation. Android Emulator (by Android Developer Studio) is a popular example.
The emulator mimics the target/mobile device processor. Then it translates its ISA into the one used by your computer, through a process called binary translation. This binary (ABI–or Application Binary Interface) can be equipped with a compatible operating system and APIs.
Capabilities: The emulator can give you virtual device instances with near-native capabilities and extended controls to adjust the devices'physical sensors, battery state, geolocation, and more.
Limitations: These near-native capabilities come with a significant performance overhead, mostly due to binary translation.

• Simulators

Simulators let you run programs that were not made for your computer’s OS. In the context of this post, ‘simulator’ refers to the iPhone and iPad simulator in XCode.
The iOS simulator sits on top of your operating system. From there, it mimics iOS and runs your app inside it. This process is viewable in an iPhone or iPad-like window.
Capabilities: The iOS simulator is significantly faster than Android emulator, purely because there’s no machine-language translation involved.
Limitations: The simulator cannot mimic real ios devices like battery states or cellular interrupts, unlike the Android emulator.
.You also cannot use the iOS simulator on platforms other than macOS. This is because the simulator needs Apple’s native Cocoa API–a massive library of frameworks–to handle GUI, runtime, and more.
Porting Cocoa to a different platform is far too much trouble. Instead, developers virtualize macOS on their computer hardware–or procure a MacBook.