Monero features an opaque blockchain (with an explicit allowance system called the @view-key), in sharp contrast with transparent blockchains used by any other cryptocurrency not based on CryptoNote. Thus, Monero is said to be "private, optionally transparent".
Every Monero address has a private viewkey which can be shared.By sharing a viewkey, a person is allowing access to view every incoming transaction for that address.However, outgoing transactions cannot be reliably viewed as of June 2017. Therefore, the balance of a Monero address as shown via a viewkey should not be relied upon.