More often these terms are interchangeably used in QA testing. Just to draw the line, here’s a definition which I agree with Goranka Bjedov on her Google Techtalk regarding “Using Open Source Tools for Performance Testing”

Performance – measuring how quickly the system responds to a work load. Response time tells everything,  proportional to its database, network infrastructure etc.

Stress – measuring  when and how the system fail and recover under an extreme load condition.

Load – measuring how the system behaves on a particular load (extremely high and low) over a prolonged period of time.