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. This issue often arises in programming languages like C and C++, which do not automatically check the size of data being written. Attackers can exploit buffer overflows to inject malicious code, leading to unauthorized access or control over a system. Proper coding practices and tools can help prevent these vulnerabilities.