Title, Description and Comments are the three core text fields in cards.
The title is the name of the card and can be seen on it when viewing the card on a board or in a backlog. Along with tags, the title is also the only thing that can be searched for.
Description lets you further explain the card and comments allow users to collaborate on it. These two text fields also allow for some formatting, just click Formatting help and you will find all the formatting options available.