The main idea behind this is to access to a network and start hacking (ethical hacking such as his lecture course Ethical Hacking – Binary Auditing & RCE) to test the security against hacker attacks, malware and virus. Old versions of software including Apache, MySQL, PHP, FTP and SSH daemons are included as well as the tools needed to exploit them such as GCC, GDB, NASM, strace, ELF, Shell, DDD, LDasm, and LIDa.
DVL is free to download, but be warned this is a highly exploitable version of the Linux and use it just for educational and testing purpose not has a main computer or working computer. If you want to download it just follow the previous link, it's only 1.8GB the ISO can be used as a Live CD, or installed as a virtual machine using a package like VirtualBox or VMWare.
Source: Damn Vulnerable Linux