SQL injection is an attack where malicious code is passed to an SQL Server for execution. The attack can result in unauthorized access to confidential data, or destruction of critical data.
Before you try to read the methods below, realize that this should only be a concern for PHP developers and the like. If you are using a database driven program (e.g. WordPress, Joomla, OSCommerce), then all you need to do is upgrade your programs to the latest version available.
One way to prevent injections is to escape dangerous characters (i.e. backslash, apostrophe and semicolon). In PHP, it is typical to escape the input using the function mysql_real_escape_string before sending the SQL query. Example:
A parameterized query uses placeholders for the input, and the parameter values are supplied at execution time.
Advanced: In PHP version 5 and above, there are multiple choices for using parameterized statements; the PDO database layer is one of them. There are also vendor-specific methods; for example, MySQL 4.1 + used with the mysqli extension.