Much of what you hear in popular songs these days can be acreditted to plugins and processing in the studio. Many crazy sounding, and even realistic sounding instruments used in the box are all thanks to the use of plugins. However they are not only used for just virtual instruments! Plugins can be devided easily into two categories; inserts and sends.
An insert is a plugin that processes 100% of the signal and directly attenuates what sound you hear coming from the output. Plugins like equalization and compression are the two most common examples of insert plugins. Because we are using these tools to shape or alter the signal to our preference, we make sure they effect all of the signal, and not just part of it.
Sends on the other hand give the user more leway on how much of the signal will be processed or altered. This can be desired when dealing with plugins such as reverb or delay, in which proccessing 100% of the signal with these effects can be very overwhelming and ends up sounding worse than if you only effected say 50% of the signal.
The proper use of plugins in the right hands can take moderately good tracks and turn them into beautiful tracks, if the user knows how to control and manipulate the plugins.