What Is a Bicycle?

Components

"Components" are all the parts that attach to the frame and fork.

The quality of components has improved tremendously over the years. If you are buying a "previously-loved" bike, it might be a good idea to upgrade its components over time.

Design & Engineering
- Better components work better
- Have more adjustments and/or are easier to adjust
- Are better thought out
Preferences

Besides locomotion, bicycles have to perform a host of other functions (braking, shifting, etc.).
What approach to these functions works best for you depends on fit (your proportions), and ergonomics (what mechanism feels/works best for you).
Aesthetics (Bling Factor)

There is a mind-boggling number of after-market parts available for every single part of the bicycle.

No fenders? Polycarbonate? Hammered steel? Bentwood?

The choice is yours ...