A web application firewall (WAF) is an applican firewall, delpoyed to secure web application servers and infrastructure from internet-based malicious attacks and security breaches from outside networks.
WAF or Web Application Firewall is a pre-installed third party application firewall employed between the internet and web servers. The firewall is equipped to screen and so as to filter out the messages. The firewall once configured on the system scans for any malicious attempts like DdoS, XML injection, SQL injection attacks through requests and messages and obstructs any such suspicious messages and requests.
Web Application Firewall can either be an independant hardware device or a cloud-based software.