- User threads - Kernel Threads Linux - task_struct has mm_struct (&mm) points to another task_struct (mm_struct) points to the same then its a thread and another mm_struct it's process. kernel threads - share address space. there is a thread count associated with the processes :- fork() - creates a process not associated with a process. When you do getpid() - returns processid Different os has different concepts for threads: fork means create a copy of task_struct and what it means the process or thread_struct which forks gets copied. gettid - gives thread id. linux has a concept of task group leader so the id of the task group leader is returned as pid for the process task_pid refers to the tasks pid. Lightweight process is basically kernel thread. main issue with the use of thread is synchronization t1 and t2 have both the threads int x; Thread gets created in Linux using Clone (sys_clone) one of the flags which is more interesting CLONE_VM (which shares memory between processes or threads). x++ corresponds to LD x, ADD X, 1 , WR X