As others have answered, Activity Monitor in
/Applications/Utilities/ is the most direct equivalent to the Windows Task Manager, but there are other options..
I mainly use iStat Menus to work out which application is using up all the CPU time..
Instead of having to launch Activity Monitor (which takes a few seconds to start), I just click the little CPU menu bar item, and it lists the current top processes:
You can also use the "top" command instead of Activity Montior (it's quicker to launch also). Just run the
top command in a terminal:
I have an alias
ltop which launches top, ordered by CPU usage, with a few flags to reduce it's CPU usage (reduces it's accuracy with regards to memory usage, but uses about 2% CPU instead of about 10-12% by default):
alias ltop='top -F -R -t -o cpu'