1. Literal... that is it ... like if you physically point something out.

2. As others mention... in frustration we say, "that's it, i have had enough"

3. I also think we sometimes use it like the french "voilá" ... as if to say "you see, that's it! .. a kind if emphatic positive confirmation of some point.

That is just off the top off my head...

Simone,  You could say "That's it" at the end of a conversation as a finisher. 

Additionally the phrase could be used as a command - "That's it!" - as in, "No more!"

Or- "That's it!" could also be an 'aha moment', when a solution to a problem is found.

@darren potter: yes, great suggestions - you can use it at the beginning of a sentence but note that your intonation will change.

You can also use it to confirm something:  "That's it!  That's what I was thinking / that's what I was going to say!"

or "That's it!  you guessed it!"

It's informal so no, I wouldn't use it to finish a speech.  And you wouldn't use it for an introduction as 'that's it' means 'I'm finished'.

You can use this phrase, usually after a series of events, to signal that you are done, you are finished or that you have nothing further to say.

e.g., I went to the store, I bought some items and then I took the bus home.  That's it. (e.g., I just did these 3 things and nothing else).

Hope that helps a little.

After a few more corrections like this on italki, I'm going to say to myself 'that's it' and then I'm going to sleep :)