In performance engineering, what is the difference between load testing and performance testing when evaluating how a system behaves under user demand?

Introduction / Context:
Performance related questions are very common in interviews, because modern applications must handle many users and large data volumes. This question checks whether you can distinguish between the narrow concept of load testing and the wider umbrella term of performance testing. A clear understanding helps in planning realistic non functional test strategies.

Given Data / Assumptions:
- The system under test is a multi user application such as a web or enterprise system.
- Load testing is one type of performance related test.
- Performance testing includes several categories like load, stress and endurance testing.
- No calculation is required; the answer is conceptual.

Concept / Approach:
Performance testing as a discipline aims to measure how fast and stable a system is under different operating conditions. Typical metrics are response time, throughput, resource utilisation and error rate. Load testing is a specific scenario in which the system is exposed to a particular number of concurrent users or transactions that represent normal or peak expected usage. Other performance subtypes, such as stress testing, push the system beyond expected limits, while endurance testing checks behaviour under sustained load over long periods.

Step-by-Step Solution:
Step 1: Recall that performance testing is the overall activity of measuring and tuning the speed and efficiency of a system. Step 2: Recognise that load testing is normally defined as testing with a specific, realistic user or transaction load. Step 3: Review each option and discard those that clearly do not talk about response time, throughput or user load. Step 4: Option a states that load testing focuses on expected or peak load, and that performance testing covers a wider range of conditions and metrics. Step 5: Based on these definitions, option a best captures the true relationship between the two terms.

Verification / Alternative check:
To verify, imagine a test plan for a new web application. The plan contains response time measurements at different user loads, stress tests at double the expected traffic, soak tests overnight and tuning activities. This entire set is called performance testing. The specific scenario where the system is run at expected production load is the load test. This scenario matches option a, confirming it as correct.

Why Other Options Are Wrong:
Option b is unrelated to standard definitions and confuses functional aspects with non functional testing. Option c is incorrect because industry literature clearly treats load testing as a subset of performance testing, not as a complete synonym. Option d reduces the concepts to single technical metrics and therefore fails to represent how these tests are actually designed.

Common Pitfalls:
Testers sometimes report that they have done performance testing when they have only run one simple load scenario. Another pitfall is designing unrealistic load profiles that do not match production behaviour, which gives misleading results. It is important to model user behaviour carefully and include combinations of load, stress and endurance to get a full picture of system performance.

Final Answer:
Load testing focuses on system behaviour under expected or peak user load, whereas performance testing is a broader term that measures response time, throughput and resource usage under various conditions, including load, stress and scalability tests.