The ISTQB define load testing as:
A type of performance testing conducted to evaluate the behavior of a
component or system with increasing load, e.g. numbers of parallel users and/or numbers
of transactions, to determine what load can be handled by the component or system
Hmmm, sounds a little like Stress testing! But load differs because in stress testing we are trying to evaluate the extent to which the system keeps working when we subject it to extreme loads. So they are obviously related, because in order to stress test something we usually need to "load" it.
We can also say that in load testing we are not actually trying to break the system, whereas we might in stress testing. In Load testing we are trying to determine how the system behaves under normal conditions, as well as under peak conditions. Stress testing is not concerned with normal behavior, other than to compare to the results of stressed behavior.
And not to forget - Load testing is a type of "non-functional" test. When I think of load testing I usually think of numbers of users and transactions because my background in eCommerce and retailing demanded this. However there are other measures we could use for loading such as downloading multiple files from the internet.
In the tests I performed, my primary goal was to determine how the system performed when loaded at it's expected number of users and transactions during normal days activities, and then to simulate peak scenarios such as during promotions or Christmas. This information is very valuable to the business as a prediction of reliability and performance.
Don't underestimate the time it can take to perform Load testing. There are many tools out there to help us simulate these conditions, so it makes good sense to really do your research before undertaking your load testing.