To create heavy user load to the server and estimate the 
server performance is call load tesing. where as take one 
by one user load to the server and estimate the server 
performance is called stress testing.

time consume by load testing is very low.

time consume by stress testing is very high because we take 
the load one by one.

if anyting wrong then plz inform me.

Testing an application under heavy load,such as testing of a
website under a range of loads to determine at what point
the system's time degrades or fails.
Beyond the expected or increase the load is called stress
testing.

Simply

if u r giving inputs beyond limit to test that will be 
recognised as strss testing

and if we r giving inputs within limit to test will be 
recognised as load testing.

max no of users  testing the application is called load 
testing.max no of users testing the app at atime is called 
stress testing

Testing application to check how it will behave with 
increasing number of users/ For perticular scenario is 
called LOAD TESTING

but when you start testing the system when system is so 
much loaded such that it fails to execute...

For example- you test online banking Application that can 
handle 10000 transactions/Sessions for perticular function

when you start testing with 100, 1000, 4000 etc. to analyze 
time taken, performance, system issues is called LOAD TEST

when you keep doing this you may find that system becomes 
slow with increasing load and when reaches 10000+ it fails 
to function further, but it should take care of all the 
failed transactions/ other functionalities/ related pages 
etc. in best way can be considered as a STRESS TEST (this 
is just a simple example...)