A private information retrieval (PIR) protocol is a protocol that allows a user to retrieve an item from a server in possession of a database without revealing which item is retrieved.
An oblivious transfer (OT) protocol is a type of protocol in which a sender transfers one of the potentially many pieces of information to a receiver, but remains oblivious as to what piece (if any) has been transferred.
As the Wikipedia said, PIR is a weaker version of 1-out-of-n oblivious transfer, but I do not really know why is a weaker version.