First of all, both testing are used for testing the 
performance of the application.More generally we can say 
that these are the techniques to perform PERFORMANCE 
TESTING.
In the LOAD testing we check the application for the volume 
of users at the peak level. For example say a application 
can afford 500 users at the peak time and this time remains 
for 5 hrs.then we'll test the application with 500 virtual 
users for 5 hrs. Keep it remember that the this is 
performed in steps i.e. firt we test the application for 
5,10,50,100,....,500 virtual users. And in this way we test 
the application for specified number of users.
In the STRESS Testing we test the application for beyond 
the specified limit(number of users). And check what the 
degradations take place before going to fail,how the system 
starts behaving????
LOAD and STRESS testing are almost two synonymous term.
The only difference is that in Load Testing we test for the 
SPECIFIED LIMIT and in Stress Testing for BEYOND THE LIMIT.

I think some of users confused with some terminology used 
in industry ?

I) Load test and stress test? Both are same???
One of the most common, but unfortunate misuse of 
terminology is treating "load testing" and "stress testing" 
as synonymous. The consequence of this ignorant semantic 
abuse is usually that the system is neither properly "load 
tested" nor subjected to a meaningful stress test.

1. Stress testing is subjecting a system to an unreasonable 
load while denying it the resources (e.g., RAM, disc, mips, 
interrupts,etc.) needed to process that load. The idea is 
to stress a system to the breaking point in order to find 
bugs that will make that break potentially harmful. The 
system is not expected to process the overload without 
adequate resources, but to behave (e.g., fail) in a decent 
manner (e.g., not corrupting or losing data). Bugs and 
failure modes discovered under stress testing may or may 
not be repaired depending on the application, the failure 
mode, consequences, etc. The load (incoming transaction 
stream) in stress testing is often deliberately distorted 
so as to force the system into resource depletion.

2. Load testing is subjecting a system to a statistically 
representative (usually) load. The two main reasons for 
using such loads is in support of software reliability 
testing and in performance testing. The term "load testing" 
by itself is too vague and imprecise to warrant use. For 
example, do you mean representative 
load," "overload," "high load," etc. In performance 
testing, load is varied from a minimum (zero) to the 
maximum level the system can sustain without running out of 
resources or having, transactions suffer (application-
specific) excessive delay.

3. A third use of the term is as a test whose objective is 
to determine the maximum sustainable load the system can 
handle. In this usage, "load testing" is merely testing at 
the highest transaction arrival rate in performance testing

Suresh.Chaluvadi

Load testing is also called as "scalability testing".scale 
means number of cucurrent users.to test the application 
build with maximum customer expected load is called load 
testing
testing the application with more than customer expected 
load is called stress testing .In general many companies 
test with customer expected load+5% of the customer 
expected load

I agree with suresh.

Stress testing is subjecting a system to an unreasonable 
load while denying it the resources needed to process that 
load.

Load testing is subjecting a system to an usual load 
without running out of resources.

To test how much load is applied on Server/CPU and response
time of server for different configuration of load is known
as Performance testing. For conducting performance testing,
we use three techniques:

1.LOAD TEST --> Estimating the performance of Server/CPU
under customer expected load is called Load Test.

2.STRESS TEST --> Estimating the performance of Server/CPU
under maximum load or peak load is called Stress Test.

3.VOLUME TEST --> Estimating the response time of Server/CPU
applying more than maximum load or peak load is called
volume test.