进程是执行中的程序，它不仅仅包含程序（代码段），还包括当前的活动,通过程序计数器的值和寄存器的内容来表示，另外进程还包括堆栈段（包括临时数据，如函数参数、返回地址和局部变量）和数据段（包括全局变量）。进程还可能包括堆，是在进程执行过程中动态分配的内存，大致如下图：
进程是通过进程控制块（PCB）来描述的，它包含了许多与进程相关的信息：
在linux中使用结构体task_struct来描述一个进程，如下基于linux2.6.11的源码（linux-2.6.11/include/linux/sched.h）：
struct task_struct {
volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */
struct thread_info *thread_info;
atomic_t usage;
unsigned long flags; /* per process flags, defined below */
unsigned long ptrace;
int lock_depth; /* Lock depth */
int prio, static_prio;
struct list_head run_list;
prio_array_t *array;
unsigned long sleep_avg;
unsigned long long timestamp, last_ran;
int activated;
unsigned long policy;
cpumask_t cpus_allowed;
unsigned int time_slice, first_time_slice;
#ifdef CONFIG_SCHEDSTATS
struct sched_info sched_info;
#endif
struct list_head tasks;
/*
* ptrace_list/ptrace_children forms the list of my children
* that were stolen by a ptracer.
*/
struct list_head ptrace_children;
struct list_head ptrace_list;
struct mm_struct *mm, *active_mm;
/* task state */
struct linux_binfmt *binfmt;
long exit_state;
int exit_code, exit_signal;
int pdeath_signal; /* The signal sent when the parent dies */
/* ??? */
unsigned long personality;
unsigned did_exec:1;
pid_t pid;
pid_t tgid;
/*
* pointers to (original) parent process, youngest child, younger sibling,
* older sibling, respectively. (p->father can be replaced with
* p->parent->pid)
*/
struct task_struct *real_parent; /* real parent process (when being debugged) */
struct task_struct *parent; /* parent process */
/*
* children/sibling forms the list of my children plus the
* tasks I'm ptracing.
*/
struct list_head children; /* list of my children */
struct list_head sibling; /* linkage in my parent's children list */
struct task_struct *group_leader; /* threadgroup leader */
/* PID/PID hash table linkage. */
struct pid pids[PIDTYPE_MAX];
struct completion *vfork_done; /* for vfork() */
int __user *set_child_tid; /* CLONE_CHILD_SETTID */
int __user *clear_child_tid; /* CLONE_CHILD_CLEARTID */
unsigned long rt_priority;
unsigned long it_real_value, it_real_incr;
cputime_t it_virt_value, it_virt_incr;
cputime_t it_prof_value, it_prof_incr;
struct timer_list real_timer;
cputime_t utime, stime;
unsigned long nvcsw, nivcsw; /* context switch counts */
struct timespec start_time;
/* mm fault and swap info: this can arguably be seen as either mm-specific or thread-specific */
unsigned long min_flt, maj_flt;
/* process credentials */
uid_t uid,euid,suid,fsuid;
gid_t gid,egid,sgid,fsgid;
struct group_info *group_info;
kernel_cap_t cap_effective, cap_inheritable, cap_permitted;
unsigned keep_capabilities:1;
struct user_struct *user;
#ifdef CONFIG_KEYS
struct key *session_keyring; /* keyring inherited over fork */
struct key *process_keyring; /* keyring private to this process (CLONE_THREAD) */
struct key *thread_keyring; /* keyring private to this thread */
#endif
int oomkilladj; /* OOM kill score adjustment (bit shift). */
char comm[TASK_COMM_LEN];
/* file system info */
int link_count, total_link_count;
/* ipc stuff */
struct sysv_sem sysvsem;
/* CPU-specific state of this task */
struct thread_struct thread;
/* filesystem information */
struct fs_struct *fs;
/* open file information */
struct files_struct *files;
/* namespace */
struct namespace *namespace;
/* signal handlers */
struct signal_struct *signal;
struct sighand_struct *sighand;
sigset_t blocked, real_blocked;
struct sigpending pending;
unsigned long sas_ss_sp;
size_t sas_ss_size;
int (*notifier)(void *priv);
void *notifier_data;
sigset_t *notifier_mask;
void *security;
struct audit_context *audit_context;
/* Thread group tracking */
u32 parent_exec_id;
u32 self_exec_id;
/* Protection of (de-)allocation: mm, files, fs, tty, keyrings */
spinlock_t alloc_lock;
/* Protection of proc_dentry: nesting proc_lock, dcache_lock, write_lock_irq(&tasklist_lock); */
spinlock_t proc_lock;
/* context-switch lock */
spinlock_t switch_lock;
/* journalling filesystem info */
void *journal_info;
/* VM state */
struct reclaim_state *reclaim_state;
struct dentry *proc_dentry;
struct backing_dev_info *backing_dev_info;
struct io_context *io_context;
unsigned long ptrace_message;
siginfo_t *last_siginfo; /* For ptrace use. */
/*
* current io wait handle: wait queue entry to use for io waits
* If this thread is processing aio, this points at the waitqueue
* inside the currently handled kiocb. It may be NULL (i.e. default
* to a stack based synchronous wait) if its doing sync IO.
*/
wait_queue_t *io_wait;
/* i/o counters(bytes read/written, #syscalls */
u64 rchar, wchar, syscr, syscw;
#if defined(CONFIG_BSD_PROCESS_ACCT)
u64 acct_rss_mem1; /* accumulated rss usage */
u64 acct_vm_mem1; /* accumulated virtual memory usage */
clock_t acct_stimexpd; /* clock_t-converted stime since last update */
#endif
#ifdef CONFIG_NUMA
struct mempolicy *mempolicy;
short il_next;
#endif
};当代码段加载到系统中进行执行的时候就创建了一个进程，当然，系统是可以运行多个进程的，这些进程会被加入到作业队列中，驻留在内存中就绪的、等待运行的进程保存在就绪队列中。该队列通常使用链表实现，其头节点指向链表的第一个和最后一个PCB块的指针，每个PCB块包括一个执行就绪队列下一个PCB块的指针域。
线程是CPU使用的基本单元，由线程ID、程序计数器、寄存器集合和栈组成。它与属于同一进程的其他线程共享代码段、数据段和其他操作系统资源，如打开文件和信号。
有两种方法来提供线程支持：用户层的用户线程或内核层的内核线程。用户线程受内核支持而无须内核管理；而内核线程由操作系统直接支持和管理。在某个特定操作系统中的用户线程和内核线程之间必然有一种对应关系，三种常用的对应关系模型是：
多对一模型、一对多模型、多对多模型。
线程库是为编程人员提供创建和 管理线程的API，主要有两种方式来实现，第一种方法是在用户空间提供一个内有内核支持的库，此库的所有代码和数据结构都在用户空间，调用库中的函数只是导致了用户空间的本地调用而非系统调用。第二种方法是执行一个由操作系统直接支持的内核级的库。此时，库的所有代码和数据结构都在内核空间，每次调用都是系统调用。
目前使用的三种线程库是：POSIX Pthread、Wind32、Java。Pthread作为POSIX标准的扩展，可以提供用户级和内核级的库。win32线程库是适用于Windows操作系统的内核级线程库。Java线程API允许线程在Java程序中直接创建和管理。然而由于大多数JVM实例运行在宿主操作系统上，所以Java线程API通常采用宿主系统上的线程库来实现。这意味着，在Windows系统上使用win32API实现，而在Unix和Linux系统中采用Pthread。