Final Fantasy XIII-2 is the sequel to Final Fantasy XIII. Final Fantasy Versus XIII, on the other hand, is a spin-off of the series.

All of those games are considered part of the Fabula Nova Crystallis series.
http://finalfantasy.wikia.com/wiki/Fabula_Nova_Crystallis:_Final_Fantasy

Edit: Just to be more clear. 4 games are developed under the Fabula Nova Crystallis: Final Fantasy series.
- Final Fantasy XIII and its sequel, XIII-2.
- Final Fantasy Versus XIII
- Final Fantasy Type-0 (used to be called Agito XIII)

0 wii game.

Its not a spin off.

Its a seperate game, full budget and all.

<_<

I hate when people say it's a spin-off because it's not, it's a new game with new story and characters but has the same myth of FNC , Versus development started at the same time as XIII .

I hate you so much .

It's a spin-off or part of the Fabula Nova Crystallis series. That's what I meant.

Read above. I apologize for not  being clear.

I love you so much.

I love when people say it's not a spin-off because it's the truth

I love you so much XD

@Darth and VXIII
I editted my original post so you guys won't harass me anymore.

A spin off is a lower budget side story to a given title.

XIII-2 could be classified as a spin off, as X-2 could too.

But Versus XIII is a BIG BUDGET (bigger budget than XIII for sure) entry with an alternate story

It is NOT a spin off.

He's such a good boy relax , he edited his post XD

It is this mentality that has me worried about its sales.

It needs to be rebranded as FFXV or it will forever remain a spin off in the eyes of people, and with XIII-2 coming people are going to get so confused between all these XIII's.

And SE will do that, as they did for Type-0, I am sure of it.

@Fab_GS

Kay