Kotlin is a statically typed, cross-platform, general-purpose programming language developed by JetBrains. It was officially introduced in 2011 and has gained significant popularity as an alternative to Java for Android app development, as well as for other application domains.

Key features and concepts associated with Kotlin include:

1. Conciseness: Kotlin is known for its concise and expressive syntax, which reduces boilerplate code compared to languages like Java. This makes code more readable and less error-prone.
2. Safety: Kotlin places a strong emphasis on safety and eliminates common programming errors by design. Features like nullable and non-nullable types, type inference, and safe call operators help prevent null pointer exceptions and other runtime errors.
3. Interoperability: Kotlin is fully interoperable with Java, meaning you can use Kotlin and Java code together within the same project seamlessly. This makes it easy to adopt Kotlin gradually in existing Java codebases.
4. Null Safety: Kotlin’s type system distinguishes between nullable and non-nullable types. This helps developers avoid null pointer exceptions by ensuring that null values are handled explicitly when necessary.
5. Functional Programming: Kotlin supports functional programming paradigms, including first-class functions, lambda expressions, and higher-order functions. It allows developers to write concise and expressive code for tasks like filtering, mapping, and reducing collections.
6. Extension Functions: Kotlin allows developers to extend existing classes with new functions without modifying their source code. This feature enhances code readability and maintainability.
7. Smart Casts: Kotlin’s smart casting mechanism automatically casts types when certain conditions are met, reducing the need for explicit type casting.
8. Coroutines: Kotlin provides built-in support for asynchronous programming using coroutines. Coroutines simplify asynchronous code by allowing developers to write sequential-looking code that runs concurrently.
9. Data Classes: Kotlin’s data classes simplify the creation of classes for holding data by generating common methods like equals(), hashCode(), and toString() automatically.
10. Standard Library: Kotlin includes a comprehensive standard library that provides useful data structures, collections, and utility functions, making it easier to work with common tasks and operations.
11. Immutable Collections: Kotlin encourages immutability by providing immutable collections in its standard library. This can help ensure safer and more predictable code.
12. Multiplatform Development: Kotlin offers a multiplatform development feature that allows you to write shared code for various platforms, including Android, iOS, and web, by leveraging Kotlin/Native and Kotlin/JS.
13. Community and Ecosystem: Kotlin has a growing and active community of developers and a rich ecosystem of libraries and frameworks. Popular frameworks like Spring and Android Jetpack have adopted Kotlin as a first-class language.

Kotlin has gained particular prominence as the preferred language for Android app development. It offers many advantages over Java, such as enhanced safety, conciseness, and modern language features. However, Kotlin is not limited to Android; it is used for a wide range of application development, server-side programming, and other software development tasks across various domains.

To learn Kotlin for free go to kotlinlang.org/docs/tutorials/