Test driven development simply put is to write a test first, write the code for it, and then (eventually) pass that test. This is done in short development cycles, so the team keep writing tests and then writing the code to pass the tests on a continuous basis.
A brief video explaining TDD:
Its worth noting that BDD (Behaviour driven development) emerged from TDD and below is a good video on TDD and BDD – What Are The Key Differences?
Below is another interesting video on TDD vs BDD. In this case BDD comes out as a winner for it reduces the amount of work required, and also delivers on the required quality.
Here is a link from TechTarget to a more detailed description of BDD and an extract below from the article.
“Behavior-driven development (BDD) is a software development methodology in which an application is specified and designed by describing how its behavior should appear to an outside observer.”