To sum up process hollowing on Windows: basically, a legitimate PE binary (exe usually) is used to start a process in suspended state, after which process memory is replaced with code and data of a malicious/other binary, and the process is resumed. As far as the environment is concerned, the original legitimate binary is executing.
There is lots of information on this topic, but it all relates to Windows OS. Is this issue possible on other operating systems and how (for example, what sequence of system calls on Linux would indicate such behavior?).