CockroachDB is a distributed relational database system designed for high availability, scalability, and consistency. It is named after the resilient cockroach because it is engineered to be extremely resilient to failures. CockroachDB is based on the ideas presented in the Google Spanner paper and employs a distributed, scalable architecture model that replicates data across multiple nodes and data centers.

Written in Go, this database provides a SQL interface, making it accessible to many developers who are already familiar with SQL. CockroachDB aims to combine the scalability and fault tolerance of NoSQL databases with the relational integrity and query capability of SQL databases. It is a popular choice for applications requiring a highly available database with horizontal scalability, such as web applications, e-commerce platforms, and IoT solutions.

Horizontal scalability refers to a system's capability to handle increased workloads by adding more resources or hardware components, enhancing its performance. In contrast to vertical scalability, where performance improvement occurs by adding resources to a single node or machine, horizontal scalability scales by adding additional instances of resources that work together.

Typically, horizontal scalability means the system can distribute loads across multiple machines or servers. Cloud computing platforms are often designed to offer horizontal scalability, allowing resources to be dynamically added or removed as needed to enhance performance and availability.

An example of horizontal scalability is expanding a web server by adding more servers to better handle user requests, rather than just increasing the resources of the existing server.