As well as the explanations given by the others above (that is, the ternary operator acts as shorthand for an if-then-else construct), it has a special perlish capability that you can assign to it.

Actually, if the second and third arguments are valid "lvalues" (things what you can assign stuff to), then you can have the ternary expression on the left hand side of an assignment operation. For example, consider the following:

($cheque_account_sel ? $cheque_balance : $card_balance ) -= $payment;
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So based on a boolean flag ($cheque_account_sel) being true, the variable $cheque_balance is reduced by the value of $payment, otherwise the variable $card_balance is reduced by that amount.

Why would you want to use that? I don't know, but it is there.