Drupal is the world's most flexible Content Management System, is the world's most popular CMS for medium and large sites, plus Drupal is the first choice for saving time, money, and pain.
Drupal 7
Drupal 7 is the current version and your choice for creating a new Web site or upgrading an existing Web site.
Drupal 8
Drupal 8 is the next version of Drupal, is already in a beta version, and will be the best choice for Web sites late in 2015.
The last of the major changes to Drupal 8 are complete. Add-on modules and themes are under development. Some of the best modules and themes will be converted before the middle of 2015. You could start a new Web site project on Drupal 8 in 2015.
Architecture
Code or Database
An age old argument about development is code speed versus database flexibility. The right approach is to put data in the database and not in the code. Sometimes it is faster to put things in code. Drupal 7 let you store everything in the database, for flexibility, and cache subsets of the data for speed. Drupal 8 moves some of the cached data, the configuration settings, to external text files that are automatically converted to code for speed. The code is automatically updated when the text files are changed. You get the flexibility of settings outside of code and you get the code execution speed for free.
OOP
OOP, Object Oriented Programming, is a popular approach to developing large projects. Drupal 7 uses a flexible mixture of OOP, for those who want the best code, and a previous approach bringing in tens of thousands of developers by making changes simple. Drupal 8 moves to a mostly OOP architecture with non OOP reserved for specialist areas and interfaces to external non OOP systems. The mixed approach in Drupal 7 brings thousands of add-on modules and themes from previous versions of Drupal without requiring a huge roadblocking rewrite. The focused approach in Drupal 8 improves the code without locking out external interfaces and themes.
Not quite perfection
The architecture is not quite perfection. Architectural preferences and fashions change faster than a major application can change. The Drupal organisation tends to make better long term choices than the other brands of CMS. Drupal has enough developers to make major upgrades when the existing code is falling behind. You are not locked into a fashion from a previous decade. In return you do have to put in a major upgrade every three or four years.