OpenBSD is a derivative of the free and open source Unix operating system BSD, which was first developed at the University of California, Berkeley. The OS is probably best known for its mascot, the puffer fish named Puffy, and also for its very popular sub-project OpenSSH, which is used on many of the servers around the world.
OpenBSD is known for its focus on security and code purity. The project maintainers make a conscious effort to go the extra mile and auditing and verifying the code security. Many security firms and major companies have used it specifically for its security usability in firewalls, intrusion-detection systems, and other security devices.
On servers, OpeBSD includes many of the necessary applications for server use, including web server, mail server, and firewall tools. OpenBSD relies on the OpenBSD packages and ports system for its package management. Users thus have the option of installing binary packages or using ports to build the packages from source and run fully optimized.