SQL injection usually occurs when you ask a user for input, like their username/userid, but the user gives ("injects") you an SQL statement that you will unknowingly run on your database. For example:
Look at the following example, which creates a SELECT statement by adding a variable (txtUserId) to a select string. The variable is fetched from user input (getRequestString):
txtUserId = getRequestString("UserId");
txtSQL = "SELECT * FROM Users WHERE UserId = " + txtUserId;
If user enter something like this: "100 OR 1=1" then the SQL statement will look like this:
SELECT * FROM Users WHERE UserId = 100 OR 1=1;
The SQL above is valid and will return ALL rows from the "Users" table, since OR 1=1 is always TRUE. A hacker might get access to all the user names and passwords in this database.