Operations are long length phased activities that require 8 or 16 players depending on the settings of the phase, and they generally take 1-2 hours to complete. Each operation has at least some story component, with several of them advancing the plot of the Dread Masters arc. There are quest givers scattered throughout the Gav Daragon or Ziost Shadow fleet ships, where a majority of the operations are accessed from.
Because of the number of mechanics I will not be discussing how to clear operations in this guide. If you are interested in learning about the different operation bosses and how to clear them I suggest heading over to Merlyn's operation guide (https://sites.google.com/view/merlyn-swtor/home) for more information on preparing yourself for operations and surviving these more difficult encounters.