put_user() and get_user() are used to get and put single values (such as an int, char, or long) from and to userspace. A pointer into userspace should never be simply dereferenced: data should be copied using these routines. Both return -EFAULT or 0.
copy_to_user() and copy_from_user() are more general: they copy an arbitrary amount of data to and from userspace.
Unlike put_user() and get_user(), they return the amount of uncopied data (ie. 0 still means success).
The functions may sleep implicitly. This should never be called outside user context (it makes no sense), with interrupts disabled, or a spinlock held.