What is the 'config_t'?

The code I'm working with:
http://www.arduino.cc/playground/Code/EEPROMWriteAnything

I know it works, but if I can understand HOW, it'll be way better. So...
There's no mention of 'struct' in the reference pages, but I understand that it makes something like a group variable. What is the 'config_t'? I couldn't find much googling it.
struct config_t
{
long alarm;
int mode;
} configuration;

It's called a "tag". It's somewhat like a typedef. It allows you to create multiple instances of the structure...

struct config_t ConfigLoadedFromStorage;

struct config_t UserModifiableConfig;

Ah, I got, after looking up 'tag', this is very clear:
http://www.exforsys.com/tutorials/c-language/c-structures-and-unions.html

...so it's optional.

...no, it doesn't seem to be optional like that webpage says, at least it doesn't compile without it.

It is optional. This compiles without error...

struct
{
    long alarm;
    int mode;
} configuration;

void setup( void )
{
}

void loop( void )
{
}