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LoadRunner is a tool, or more accurately a collection of tools, which facilitates in
performing load testing on a system. LoadRunner is used to create a load or stress on a system, and can be used to measure the responsiveness of the system to that load. The key concept behind LoadRunner is the use of virtual users, or vusers.

What is "Virtual Users"?

To perform a load test, a system needs many users to access the system concurrently, often one hundred or more. Performing such a test with people is a logistical nightmare. When considering that the test cycle is generally iterative, this problem is compounded.
LoadRunner overcomes this problem through the use of vusers. A vuser performs the actions that a real user would perform, except that a vuser is controlled by LoadRunner, and the actions are performed automatically.
Vusers are created to perform specific transactions in the system. Once created a vuser can be replicated as many times as required, thus producing an effective load as large as is required. With the exception of GUI vusers, virtual users don’t use the client to perform their actions; they take the place of the client. This allows in many vusers to be run on one workstation, as the overhead associated with the client is not required.
This does mean, however, that non-GUI vusers do not test the performance associated with the client side of a system, only the network and server associated performance.