An open sourced version of the system that Google uses internally on most of its projects, the Protocol Buffers stack is an IDL similar to Thrift. One difference is that Thrift includes network client and server code in its generated stubs, whereas protobuf limits its scope to pure serialization and deserialization. The biggest differentiator between the two projects is probably their developer base.
Though the code is open source, Google is the main contributor and driver for Protocol Buffers, whereas Thrift is more of a classic crowd-sourced project. If your requirements skew towards stability and strong documentation, Protocol Buffers is going to be attractive, whereas if you need a more open, community-based approach, Thrift will be a lot more appealing.
