Random Access Memory (RAM) is used in every piece of software (RAM). The more software you have installed on your computer, the more RAM it consumes. This is especially problematic if you're using an older computer with little RAM. If a software program refuses to load or runs slowly, the first thing you should do is close all other open programs.

Windows and mac  include tools that display this information if you want to figure out which open apps are eating your RAM:

In Windows, press Ctrl+Alt+Delete, then select Start Task Manager from the drop-down menu. Click the Processes tab in the resulting window, then the Memory menu item. This classifies all open processes according to how much RAM they use. The End Process button can be used to terminate a rogue process. 

Use the Activity Monitor onMac OS X. (called the Process Viewer in older versions of OS X). Go to Applications > Utilities to get to the Activity Monitor. After you've opened the Activity Monitor, sort the apps by RAM consumption by selecting the 'Real Memory' column