In time means before the scheduled time, or not too late. On time means at the scheduled time, or not late. An example - There's a meeting at three on Friday and I have to prepare a presentation. It's now Tuesday. So : I have to prepare my presentation in time for the meeting - but I can do it any time between now and Friday at 3. If I did it at any other time - eg Friday evening - it would be too late - there'd be no point. I have to arrive at the meeting on time - ie at 3. I mustn't be late. If I arrived at 3.10 I would be late - but not too late. its well explained by sue swift :-)

In comparing "on time" and "in time", sunita's answer is quite correct. There is another possible use for "in time" that might be mentioned. It can mean "over time" or "after a period of time". When it is used in this way, it is at the beginning of a clause or sentence, usually before the subject and always before the verb. It is used this way far less often then the other meaning, but you see it in written English. "It was a minor accident. In time, he forgot it completely." If "in time" appears after the verb, it has the meaning given by sunita. "He got there in time for the opening."

"just in time" means a little different than "in time" "just in time" means at the scheduled time, or not late (which is the same as "on time" haha)