One common utility for troubleshooting problems in OS X is Activity Monitor, which will display the system's running processes and various statistics about them, including the useful percent CPU usage and memory footprint of each. This information allows you to quickly assess whether a program is stable or if it may be experiencing an error that is impacting the system.
In the past few versions of OS X, Apple has not changed Activity Monitor much; however, once you install OS X Mavericks you will see significant changes.
For one, the program's tabs are now along the top instead of the bottom, and act as groupings for the process list's metrics in addition to showing corresponding global information. In prior versions of Activity Monitor, switching tabs would show only a small area of system-wide statistics while the process list remained static, but now the process list shows category-specific metrics for the listed running processes. For instance, the CPU tab contains the "%CPU" column for processes, but not columns for the memory used by these processes. To see this information, you would click the Memory tab. However, this separation is only a default configuration that, if desired, can be customized by selecting column options from the View > Columns menu.
While each tab acts as a custom collection of statistics for the running processes that can be customized, as with prior versions of Activity Monitor each tab also has a separate global activity chart for its respective area (CPU, Memory, Disk, Network, etc.).
One major change here is the view of the memory usage charts. If you click the Memory tab, the usage chart will show a new Memory Pressure view instead of the classic pie chart showing free, active, inactive, and wired memory. The memory pressure view changes the concept of how the system uses RAM, especially since Mavericks now includes new technologies like memory compression to optimize RAM usage.
To account for the more dynamic nature of how RAM is used in OS X, Apple has changed its memory chart to show usage in terms of a "pressure" concept. To visualize this, consider the RAM to be a tank of compressed air. As the system's activities use more RAM, more air is shoved into the tank so the pressure goes higher. As even more RAM is used, the higher pressure eventually reaches a threshold that triggers the system to write unused portions of it to disk, or force unused programs to close. As this behavior starts, the green color of the pressure graph will turn to yellow and then to red, indicating the system's RAM usage is less efficient and to optimize performance you will need to either get more RAM, or quit programs and otherwise reduce your RAM usage.
While previous Activity Monitor features allowed you to monitor Page Out behavior to see if disk storage was being used for RAM contents, you now can monitor the "Swap Used" metric to get an idea of this behavior.
The final major change to Activity Monitor is the addition of a new Energy tab, which revolves around Mavericks' focus on energy usage and battery efficiency. This tab offers several options to help determine a program's overall load on the system, and its impact on battery life.
As with the other tabs in the program, this one has its own default collection of metrics that include an Energy Impact calculation, along with statistics such as the support of App Nap, graphics card requirements, and sudden-termination support (for automatic quitting of the program if resources become low). These metrics can be used to get a more relevant feel for what programs may drain the computer's battery faster than others, and whether or not current programs are forcing the use of power-draining system components, such as the discrete graphics card.
If your system's battery life is relatively low, then instead of sorting the process list by CPU usage and trying to determine which might be the cause, first go to the Energy tab and sort the processes by Energy Impact or even Average Energy Impact. This tab only shows user applications by default (and specifically only those run in the past eight hours), so be sure to enable "All Processes" in the View menu to get a more thorough look at the system's energy efficiency.
While not specific to Activity Monitor, a related feature Apple has introduced in Mavericks is a view of application energy impact in the battery level menu. If this is enabled (done in the Energy Saver panel of System Preferences), when you activate the menu you will see a section called "Apps Using Significant Energy," which will show the ones with the highest energy impact and allow you to quit or otherwise manage them to help save battery life.