Performance Testing is a type of testing to determine how a system performs in terms of responsiveness and stability under various workload conditions.

Key aspects include:
1. **Performance Metrics:**
   - Response time
   - Throughput
   - Resource utilization
   - Scalability
   - Reliability

2. **Testing Goals:**
   - Identify bottlenecks
   - Determine system capacity
   - Validate performance requirements
   - Benchmark performance

Common types of performance tests include:

1. **Load Testing:**
   - Tests system behavior under specific load
   - Validates system performance under expected conditions

2. **Stress Testing:**
   - Tests system behavior under peak load
   - Identifies breaking points

3. **Endurance Testing:**
   - Tests system behavior over extended periods
   - Identifies memory leaks and resource issues

Example of JMeter test plan:
```xml
<?xml version="1.0" encoding="UTF-8"?>
<jmeterTestPlan version="1.2">
  <hashTree>
    <TestPlan>
      <elementProp name="TestPlan.user_defined_variables">
        <collectionProp name="Arguments.arguments"/>
      </elementProp>
      <stringProp name="TestPlan.comments"></stringProp>
      <boolProp name="TestPlan.functional_mode">false</boolProp>
      <boolProp name="TestPlan.serialize_threadgroups">false</boolProp>
    </TestPlan>
  </hashTree>
</jmeterTestPlan>
```