What's the difference between load and stress testing ?

One of the most common, but unfortunate misuse of terminology is treating “load testing” and “stress testing” as synonymous. The consequence of this ignorant semantic abuse is usually that the system is neither properly “load tested” nor subjected to a meaningful stress test.

Stress testing is subjecting a system to an unreasonable load while denying it the resources (e.g., RAM, disc, mips, interrupts, etc.) needed to process that load. 

The idea is to stress a system to the breaking point in order to find bugs that will make that break potentially harmful. The system is not expected to process the overload without adequate resources, but to behave (e.g., fail) in a decent manner (e.g., not corrupting or losing data). Bugs and failure modes discovered under stress testing may or may not be repaired depending on the application, the failure mode, consequences, etc. The load (incoming transaction stream) in stress testing is often deliberately distorted so as to force the system into resource depletion.

Load testing is subjecting a system to a statistically representative (usually) load. The two main reasons for using such loads is in support of software reliability testing and in performance testing. 

The term 'load testing' by itself is too vague and imprecise to warrant use. For example, do you mean representative load,' 'overload,' 'high load,' etc. In performance testing, load is varied from a minimum (zero) to the maximum level the system can sustain without running out of resources or having, transactions >suffer (application-specific) excessive delay. A third use of the term is as a test whose objective is to determine the maximum sustainable load the system can handle. In this usage, 'load testing' is merely testing at the highest transaction arrival rate in performance testing.