Script Injection is something like u make select queries return more information than required and passed information to the database without processing it, or something like return non privileged data. It can be applied to Sign In Pages and Search pages. For Example I have a Sign-In Page with User Name and Password Fields. and Query is like
Select * from UserTable where User ID= '" & UserID &"' and password = '" & password & "';
It seems fine at the first sight, but if a User Provides the string ' or 1=1;--, the query becomes
Select * from UserTable where User ID= '' or 1=1;--" & UserID &"' and password = '" & password & "';
It makes user to Sing In without providing any credentials
These definitions of script injections are perfectly valid - basically it involves abusing input fields provided by the application to supply malicious code. If this definition is not sufficient to your needs, I encourage you to do your own research on Google.