Stress testing is the process of determining the ability of
a computer, network, program or device to maintain a certain
level of effectiveness under unfavorable conditions. The
process can involve quantitative tests done in a lab, such
as measuring the frequency of errors or system crashes. The
term also refers to qualitative evaluation of factors such
as availability
Load Testing:it meant constantly increasing the load on the
system via automated tools. For a Web application, the load
is defined in terms of concurrent users or HTTP connections.

Stress Testing :
To evaluate a system beyond the limits of the specified 
requirements.

Load Testing : verifies a website can handle a particular 
number of concurrent users while maintaining acceptable 
response times.

Note: Load testing and stress testing are different types 
of performance testing.