Both Intruder and Repeater are tabs in Burp Suite used for testing web applications, but they serve different purposes.
Repeater is used for manually sending and modifying individual HTTP requests. It allows the user to intercept and replay requests with modifications, such as changing parameters or headers, and observe the response. This helps to test different scenarios and find vulnerabilities that may not be apparent through normal browsing.
Intruder, on the other hand, is used for automating customized attacks against web applications. It allows the user to define attack types, such as brute-force or fuzzing, and customize attack parameters, such as payloads or character sets. Intruder then sends a large number of requests with different variations of the attack parameters to test the application's security and identify vulnerabilities.