[jQuery] difference between $.post and $.ajax

[RueSe]

I know there is a lot of difference, but what are the defaultsfor
$.post? I am asking this because I am able to send with this:

$.post( "/ajax", {"data" : json });

but not with this:

$.ajax({
  type: "POST",
  url: "/ajax",
  data: json,
  processData: false,
});

The first gives me an 200 back, the second an 500?

TIA

Ruediger