Red, Green and Refactor in TDD

I’m new to this concept of TDD and usually what other developers would do is write up their functionality and write up a unit test afterwards. My question is, what’s the difference between writing test first following the red, green and refactor pattern and writing code first then testing it.

Comments

[Vlastimil Pospichal]

codecademy.com/articles/tdd-red-gr...

Test specify what you need. Must be first.

[HelpMichaelCode]

Thanks for replying and that article link you attached was helpful!