The important one is "Unnamed Structure and Union Fields"[1], in particular unnamed structs and union fields without a tag.

ISO C11 and onward allows for this:

    struct {
      int a;
      union {
        int b;
        float c;
      };
      int d;
    } foo;

In the above, you can access b as foo.b. In ISO C11, the inner struct/union must be defined without a tag. Meaning that this is invalid:

    struct {
      int a;
      union bar {
        int b;
        float c;
      };
      int d;
    } foo;

As is this: union bar { int b; float c; };

    struct {
      int a;
      union bar;
      int d;
    } foo;

-fms-extensions makes both of the above valid. You might be wondering why this is uesful. The most common use is for nicer struct embedding/pseudo-inheritance:

    struct parent {
      int i;
      void *p;
    };

    void parent_do_something(struct parent *p);

    struct child {
      struct parent;
      const char *s;
    };

    struct child *c;
    struct parent *p = (struct child *)c; // valid
    parent_do_something(p);
    c.i++; // valid

[1]: https://gcc.gnu.org/onlinedocs/gcc/Unnamed-Fields.html