Serialization is the process of turning objects into data suitable for transmission over a network or for storage. We also include deserialization, which is the reverse, under this umbrella. Serialization can have significant performance implications, not only on the network transmission speed but also on computation, as it can make up most of the expensive processing on a web server. You can read more about serialization at https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/serialization/index.
Serialization formats can be text-based or binary. Some popular text-based formats are Extensible Markup Language (XML) and JavaScript Object Notation (JSON). A popular binary format is protocol buffers, which was developed at Google. There's a .NET binary serialization format (BinaryFormatter), which is now supported in .NET Core 2.
XML has fallen out of fashion with developers, and JSON is now generally preferred. This is partly due to the smaller size of equivalent...