Typical paradigm-level code categories include procedural, object-oriented (OO/classes), and functional (FP):

• Procedural style organizes code in a top-down, linear progression through a pre-determined set of operations, usually collected together in related units called procedures.
  
• OO style organizes code by collecting logic and data together into units called classes.
  
• FP style organizes code into functions (pure computations as opposed to procedures), and the adaptations of those functions as values.
  

from You Don't Know JS Yet (book series) - 2nd Edition, Kyle Simpson, github.com.

Backwards compatibility

Code written for older versions of the language still work in new versions of the language.

Example: JS

Forwards compatibility

Code written for new versions of the language are compatible with old versions of the language (e.g. by skipping unrecognized functions).

Example: HTML, CSS