This is schemaless. Protobufs require a .proto file that describes the data being serialized. A .proto file serves a similar purpose as the `struct` keyword in C.

MessagePack uses type prefixes to describe the data. For instance an unsigned 8-bit int starts with a `0xcc` followed by a byte.

Compatibility with JSON is a characteristic of MessagePack. If you're already using JSON, you can use MessagePack as a faster/smaller substitution of JSON.

Since Protocol Buffers brings their original semantics, you must change lots of codes.

Schemaless (dynamic typing) is the most obvious difference.

I wrote about the pros/cons of schema here: http://news.ycombinator.com/item?id=2836242