A process is an instance of a program in execution, while a thread is the smallest unit of execution within a process. Processes have their own memory space, but threads within the same process share the same memory. This makes threads lightweight and faster for inter-thread communication, but more prone to concurrency issues.

A deadlock is a situation where two or more processes are unable to proceed because each is waiting for one of the other to release a resource. To prevent a deadlock, you can use methods such as: Resource hierarchy, wait-die and wound-wait schemes, and employing a timeout mechanism.

Virtual memory is a memory management technique that provides an ‘idealized abstraction’ of the storage resources that are actually available on a machine. It helps in efficiently managing the actual physical memory, enabling larger programs to run on systems with limited RAM, through techniques like paging and segmentation.

A page replacement algorithm decides which memory pages to swap out when a page of memory needs to be allocated. Common algorithms include FIFO (First In, First Out), LRU (Least Recently Used), and Optimal Page Replacement. LRU, for example, replaces the page that has not been used for the longest period of time.

System calls provide the interface between a process and the operating system. They are used for process control, file management, device management, information maintenance, and communication. Examples include open(), read(), write(), and close(). These calls transition the mode of operation from user mode to kernel mode.

Multitasking refers to the ability of an operating system to execute multiple tasks or processes simultaneously. The main advantages of multitasking include efficient use of CPU resources, improved system responsiveness, faster program execution, and better performance of concurrent applications.