Performance Testing

Load and stress testing are subsets of performance testing.

Performance testing means how best something performs under a given benchmark. For example How much time you take to run 100 meters without carrying any load (no load is the benchmark)?
Load testing is also performance testing but under various loads. The previous example if extended would be How much time you took to run the same 100mts but carrying a load of 50 kilos, 100 kilos .... ?
Stress testing is performance under stress conditions. Extending the same example as before How much time you took to run 100 meters with load or no load when a strong wind was blowing in opposite direction ?.
Extending performance, load.. testing to s/w or h/w application.

For Example
Performance : 1000 txns per minute with 1000 users concurrent
Load : How many txns when 2000, 3000, 4000 concurrent users.
Stress : How many txns when 1000, 2000, 3000 concurrent users... under conditions like server memory very low, data transmission line poor, etc...