BrowserStack is a leading software testing platform for developers to comprehensively test their websites and mobile applications across 2,000+ real browsers and devices in a single cloud platform—and at scale.
It is an automated UI testing tool that makes it fast and easy to create, maintain, and execute functional tests across desktop, web, and mobile applications. With TestComplete, you can increase test coverage and ensure you ship high-quality, battle-tested software
Selenium automates browsers. That's it! What you do with that power is entirely up to you. Primarily, it is for automating web applications for testing purposes, but is certainly not limited to just that. Boring web-based administration tasks can (and should!) also be automated as well.
Karma is not a testing framework, nor an assertion library. Karma just launches a HTTP server, and generates the test runner HTML file you probably already know from your favourite testing framework. So for testing purposes you can use pretty much anything you like.