Functional testing examines how your software, web site, or mobile app executes its requisite functions and is vital for assuring the quality of released software.
Load testing is about creating production simulations within an application or system that is as near as possible to being a finished product ready to deploy and subject to the masses.
Security testing is about making software behave in the presence of a malicious attack, even though in the real world, software failures usually happen spontaneously that is, without intentional mischief.
The goal of usability testing is to make sure that a user can complete the tasks they are expected to
complete. Usability tests if a user intuitively understands how to perform these tasks.