Answer:
Open adoption allows identifying information to be communicated between adoptive and biological parents and, perhaps, interaction between kin and the adopted person. Rarely, it is the outgrowth of laws that maintain an adoptee's right to unaltered birth certificates and/or adoption records, but such access is not universal (it is possible in a few jurisdictions - including the U.K. and six States in the U.S.). More often, open adoption is an informal arrangement subject to termination by adoptive parents who have sole authority over the child (in some jurisdictions, the biological and adoptive parents may enter into a binding agreement concerning visitation, exchange of information, or other interaction regarding the child, however).
Open adoptions are not legally enforceable. Once the papers are signed, it's up to the new parents. Most agencies don't tell the birth parents that fact until afterwards and not all parents keep their promise.
The practice of closed adoption, the norm for most of modern history, seals all identifying information, maintaining it as secret and barring disclosure of the adoptive parents', biological kins', and adoptees' identities. Nevertheless, closed adoption, may allow the transmittal of non-identifying information such as medical history and religious and ethnic background. Today, as a result of safe haven laws passed by some U.S. states, closed adoption is seeing renewed influence. In safe-haven states, infants can be left, anonymously, at hospitals, fire departments, or police stations within a few days of birth, a practice criticized by some adoptee advocacy organizations as being retrograde and dangerous.