Context conditions exist as if, then “statements” where

• The Condition is the If — if one or more of the conditions are met
• The Reaction is the Then — the behavior triggered by the presence of the context rule parameters

,

An example of why you would want to use contexts

Say you have an alert block showing upcoming downtime that needs to be placed:

• On the homepage for all page visitors to see - placed within the utility menu at the very top 
• On the client login page - because you know that a high percentage of clients enter the site there, you need the alert to be very prominent, placed at the top of the content area on the page
• On a few landing pages within the site - it does not need to be as prominent on these pages so you want the placement to be unobtrusive.

Creating context conditions to handle the three scenarios above is a good idea because: 

• It eliminates the need to use the core Blocks module for placement; performance may be improved by using Context
• Context is easy to use with an intuitive drag-and-drop UI so it is great for site builders (it is much less complex than Display Suite and Panels).
• If you change the theme, you just need to make sure to include the same regions for block placement

,

To use Context

You need to install both the Context module and Chaos Tools module (must be version 1.7 or higher). 

Included with the Context module are:

• The Context UI module that provides an administrative interface to manage contexts
• The Context Layout module that allows themes to declare and switch between templates using contexts

Required to create a contextual condition are:

• Name
• Condition
• Reaction

,

Context for Drupal 8

While the block system in Drupal 8 is amazing, you may still want to consider using the Context module to take advantage of the condition-reaction flexibility.

The Drupal 8 version of Context was ported from Drupal 7 and uses the built-in condition plugins supplied by the Plugin API. Other conditional plugins can also be used with Context.