The simple definition of load testing is
Load testing is the process of putting demand on a system and measuring its response.
Generally, we use manual and automation testing in our project which is okay for checking if the functionality is working fine and those are functional tests.
But, Just because your application will pass a functional test doesn't mean it can perform the same under a load.
Consider your application is working fine for 10 simultaneous users. Are you sure that your application will behave fine with more people using it simultaneously? If so, How many people will be able to use it simultaneously?
We can't have the same number of QAs as end-users who simultaneously check the system unless we have a budget to hire these many QAs.
Many times we hear that the site goes down due to sudden spikes in site traffic. So this type of testing is necessary, right?
So How we will ensure that our application will work well under the pressure like us :) !
We can't have actual users to test, but we can have virtual users right? who can do the requests for us simultaneously... Load testing uses this concept of creating virtual users. Through the load testing tools and libraries, we can test our application against sudden spikes in the traffic and catch the behaviour before it directly occurs in production.
Moreover, performance testing will help us scale our project. It draws a line and we should optimize our application to not cross that line.
There are many load testing tools available, Some of them are
These tools are awesome and provide many functionalities and performance statistics such as copying the user behaviour and based on that do the load testing with virtual users and so on.