what's the difference between a thread and a process

A process is a program in execution.

A process is more than the program code, which is sometimes known as the text section. It also includes the current activity, as represented by the value of the program counter and the contents of the process's registers. A process generally also includes the process stack, which contains temporary data, and a data section, which contains global variables. A process may also include a heap, which is memory that is dynamically allocated during process run time.

A process can be thought of as an instance of a program in execution. Each process is an independent entity to which system resources (CPU time, memory, etc.) are allocated and each process is executed in a separate address space. Once process cannot access the variables and data structures of another process. If you wish to access another process' resources, inter-process communications have to be used such as pipes, files, sockets etc.

A thread uses the same stack space of a process. A process can have multiple threads. A key difference between processes and threads is that multiple threads share parts of their state. Typically, one allows multiple threads to read and write the same memory (no process can directly access the memory of another process). However, each thread still has its own registers and its own stack, but other threads can read and write the stack memory.

A thread is a particular execution path of a process; when one thread modifies a process resource, the change is immediately visible to sibling threads.