The SPID column is the thread id. As far as I can tell,
LWP and SPID are the same thing; I am not sure what the value of NLWP is;
it appears to be just the count of threads. Use "ps -Lf" to see that column.
-L show threads, possibly with LWP and NLWP columns
-T show threads, possibly with SPID column
-m show threads after processes
H show threads as if they were processes
m show threads after processes
The ps man and info pages
don't tell you much about threads, probably because there's
been plenty of confusion and disagreement about how the
kernel should expose thread info, how /proc should pick it
up, etc. This old thread shows some of that. Also, Uncertainty about
implementation details probably didn't help either.