Load Testing: Its the Amount of Load your application withstands, (For Ex: The client might tell you up front that these many users are going to use the system or some rough amount of user.) What we do in Load Testing is we will put the system under those many loads and test its performance. (i.e) how fast the appilcation performs the insertion and retrivals of data with those many users.
Stress Testing: Putting the system in adverse condition, so that to check at what condition the system breaks down. (For Ex: putting the system in unfavorable condition like lowering the memory, reducing the disk space and keeping the system running for few days continuously to check the stability and withstanding ability of the application.)
Volume testing: Performance oriented system tested with high load with a lot of stress.