Why does `htop` show more process than `ps`

By default, htop lists each thread of a process separately, while ps doesn't. To turn off the display of threads, press H, or use the "Setup / Display options" menu, "Hide userland threads". This puts the following line in your ~/.htoprc or ~/.config/htop/htoprc (you can alternatively put it there manually):

hide_userland_threads=1

(Also hide_kernel_threads=1, toggled by pressing K, but it's 1 by default.)

Another useful option is “Display threads in a different color” in the same menu (highlight_threads=1 in .htoprc), which causes threads to be shown in a different color (green in the default theme).

In the first line of the htop display, there's a line like “Tasks: 377, 842 thr, 161 kthr; 2 running”. This shows the total number of processes, userland threads, kernel threads, and threads in a runnable state. The numbers don't change when you filter the display, but the indications “thr” and “kthr” disappear when you turn off the inclusion of user/kernel threads respectively.

When you see multiple processes that have all characteristics in common except the PID and CPU-related fields (NIce value, CPU%, TIME+, ...), it's highly likely that they're threads in the same process.

For me, on a more-or-less current arch linux system, ps xf shows me only the processes owned by my user ID. htop shows me all processes. Try ps -ef for a list of all processes, or perhaps ps -ejH to get a child/parent relationship listing.