What's the difference between "in time" and "on time"?

The phrases "in time" and "on time" are very similar in meaning, but English speakers use them in slightly different situations.

On time

Doing something "on time" means meeting an appointment, or meeting a time that has been set by someone. For example:

I hardly ever get to work on time.

My flight's on time, so I'll meet you at the airport at 3:30.

If you don't turn it in on time, you'll receive a 20-point deduction.

In time

Doing something "in time" means doing it before a deadline, or doing it before it becomes unavailable:

I wanted to do some kind of study abroad program, but I didn't get my applications done in time.

Oh no; it's already 9:55. Are we going to be able to make it in time?

If you make a mistake with choosing "on time" or "in time", it's not a big problem. Other English speakers might notice your mistake, but they probably won't get upset about it.