Not really a lot of difference. However, 'in time' is normally followed by clarification of what one is in time for. Also, 'in time' can be used in different circumstances as follows. I arrived in time for the meeting. I arrived on time. I danced in time with the music. (You don't say/write I danced on time with the music).

They both mean 'punctually' or 'not late'. In general we usually say 'in time for'. on time = not late The train arrived on time. I need to get to school on time. I need to get to class on time. I paid the bill on time. My invitation will arrive on time. in time for = not late for I need to get to school in time for class. We returned from our vacation in time for the new school year. The train arrived in time for that night's events. 'in time' also means 'eventually' In time, he will get used to doing a lot of homework. She will, in time, learn how to speak Spanish well.