Switching between applications that have been loaded in memory (are currently open). It generally refers to a user purposely jumping from one application to another, for example, by pressing Alt-Tab in Windows or Command-Tab in the Mac. See task switcher
Manual or Automatic
Context switching implies switching between applications that are open but dormant in the background, waiting to be resumed where they were left off. This is a manual form of task switching.
Multitasking implies automatic task switching, in which the operating system and applications cooperate in causing the computer to move from application to application in a round robin sequence. See multitasking