Probably "man proc" is the place to read! From the manual:
/proc/[pid]/task
(since Linux
2.6.0-test6)
This is a directory that contains one subdirectory for
each thread in the process. The name
of each subdirectory is the numerical
thread
ID ([tid]) of the thread (see gettid(2)). Within each of these
subdirectories, there is a set of
files with the same names and
contents
as under the /proc/[pid] directories. For
attributes that are shared by all
threads, the contents for each of the
files under the
task/[tid] subdirectories will be the same as in
the corresponding file in the parent
/proc/[pid] directory (e.g., in a
multithreaded
process, all of the task/[tid]/cwd files will have the
same value as the /proc/[pid]/cwd file
in the parent directory, since all of
the
threads in a process share a working directory). For
attributes that are distinct for
each thread, the corresponding
files under
task/[tid] may have different values (e.g., various fields
in each of the task/[tid]/status files
may be different for each thread).
In a multithreaded process, the contents of the
/proc/[pid]/task directory are not
available if the main thread has
already terminated
(typically by calling pthread_exit(3)).
So, basically understanding /proc/pid/task/ means understanding proc itself.
