Answers
Checking that a user is genuine and is who they claim to be before allowing access.
Checking that a value has been entered and that the field has not been left blank.
Checking that an input contains a required number of characters, such as at least eight.
Checking that data entered is sensible and follows a set of rules before it is accepted.
Checking an entered password by searching a stored list or database of usernames and passwords.
Checking that a number entered falls between a lower and an upper limit, such as 0 to 120.
Checking that an input matches a required format, such as a letter followed by a number.