Macs Task Manager

Macs Task Manager
See All Running Applications / Programs with Forceable Quit Menu

Hit Command+Option+Escape to summon the basic “Force Quit Applications” window, which can be thought of as a simple task manager for Mac OS X. This shows an easy to read list of all active applications running in OS X, and what’s visible here is exactly the same as what you’d see in the Dock:

Despite the windows name, you can use this to view actively running programs and apps without actually quitting them.

One obvious advantage to the Command+Option+ESC menu is that it allows you to actually take action on running apps directly, letting you force quit them if they have become errant or are shown in red font, which signifies they are not responding or are crashing. This simplified version is fairly similar to the basic “Control+ALT+DELETE” manager that exists initially in the modern Windows world.

The primary limitation with the Force Quit Menu is that, like the Dock indicators, it is limited to revealing only the “windowed apps” that are actively running in Mac OS X, thus skipping over things like menu bar items and background apps.
Processes with Activity Monitor

The most powerful app and process management utility in the OS X GUI, Activity Monitor is a powerful task manager that will reveal not only all running and active applications, but also all active and inactive processes. This includes quite literally everything running on the Mac, including the aforementioned windowed apps, and even background applications (those not visible as running in the Dock or the Force Quit menu), menu bar items, system level processes, processes running under different users, inactive processes, service daemons, quite literally anything and everything that is running as a process in OS X at any level.

The app itself resides in Finder/Applications/Utilities/, but it’s also easy to launch it through Spotlight by hitting Command+Spacebar and typing “Activity” followed by the Return key.

A way to simplify all of the information initially shown in Activity Monitor is to pull down the Process submenu and select according to what you’re looking for, like “All Processes”, “My Processes”, “System Processes”, or “Other User Processes”, among the other options. The “Search” feature is also easy to use and quite powerful, since you can start typing the name of something and it instantly updates according to which processes match the query.

Activity Monitor offers a ton of tools and options, and it’s easily the most advanced way to view extended information about all active processes without jumping into the command line. It let’s you quit processes, kill applications (kill is basically the same as force quitting), inspect and sample processes, sort processes by names, PID, user, CPU, threads, memory usage, and kind, filter processes by user and level, and also search through processes by name or character. Furthermore, Activity Monitor will also reveal general usage stats about CPU, memory, disk activity, and network activity, making it an essential troubleshooting utility for determining everything from inadequate RAM levels to diagnosing why a Mac could be running slow based on the myriad of other possibilities.

As an added bonus, you can also keep Activity Monitor running all the time and turn it’s Dock icon into a live resource usage monitor to see what CPU, RAM, disk activity, or network activity are up to on a Mac.
Advanced: View All Running Processes with Terminal

Delving into the command line, you can use a few more advanced tools to view every single process running on the Mac, ranging from basic user-level apps to even the tiny daemons and core system functions that are otherwise hidden from Mac OS X’s general user experience. In many ways, these tools can be thought of as command line versions of Activity Monitor, and we’ll focus on two in particular: top and ps.
top

Top will show a list of all running processes and various statistics about each process. It’s usually most helpful to sort by processor usage or memory usage, and to do that you’ll want to use the -o flag:

Sort top by CPU:
top -o cpu

Sort top by memory usage:
top -o rsize