A buffer overflow occurs when a program writes more data to a block of memory, or buffer, than it can hold. This excess data can overwrite adjacent memory, potentially corrupting data, crashing the program, or creating security vulnerabilities. Attackers often exploit buffer overflows to inject malicious code into a program. By carefully crafting the input, they can manipulate the program's execution flow, leading to unauthorized access or control over the system. This makes understanding and preventing buffer overflows crucial for software security, especially in languages like C and C++ that do not automatically manage memory.