What is a theme

In the relation of Drupal, the term "theme" means a collection of inter-related files that are responsible for the look and feel of a Drupal website One of the key advantages of Drupal's architecture is the ability to obstruct and override the output of the default system without having to make changes to the core files.

Obstruct and override is very useful in case of forms, during the process of getting data from its raw form to its final displayed form, Drupal provides several opportunities for you to affect the output. The Drupal system relies on a pre-determined hierarchy to determine the order in which files are processed. we can use this as advantage by creating files of our own and injecting them into the process, thereby taking precedence over the default files. Never ever modify the files in the core

What are Regions

Regions are, placeholders inside the page layout where a site administrator can put functional output. This is most frequently done by assigning blocks to the region desired.If there is no output assigned to the region then they won't be occupying any space on the screen. Normally, regions will collapse when empty.Regions are created in theme .info file and they cannot be created by administratorfrom within the admin interface, without having to write any code.Blocks are normally assign to the region where they want the output to appear.

What are Blocks:

Blocks are typically the visual parts of the sites, content is dispalyed through the the block on site. Blcks can be created in three ways

1. When the site administrator activates a module and that module produces visual output. These types of block become available under the structure -> block.
2. The block can be manually created by the administrator and displayed from within the Block Manager.
3. Custom block can be created by the developer in a module. and when the modulke is enable the block will be automatoically available under the structure -> block.