What are the basic difference between online and real time operating system?

In a simple timeshare operating system all processes receive a time "slice", meaning that a process can run for a specified amount of time, typically measured in fractions of a second.

In a simple realtime operating system, all processes are assigned a process "priority" If a process is ready to run and it has a higher priority than other processes, the process has control of the CPU until the process gets to a point that it is no longer "ready". That can mean that the process is waiting for I/O or for the completion of some event. If multiple process with higher and equal priority are "ready", the CPU is typically shared between them, using some kind of timesharing.

Most of todays operating systems use a blending of the concepts.