Card templates tell Anki which fields should appear on the front and back of your card, and control which cards will be generated when certain fields have text in them. By adjusting your card templates, you can alter the design and styling of many of your cards at once.
Card templates are covered in some of the intro videos:
You can modify card templates by clicking the "Cards..." button inside the editing screen.
For older Anki versions, on the top left is the front template, on the bottom left is the back template, and in between them is the card styling section. For Anki versions 2.1.28+ the front, back, and styling are no longer shown at the same time. You can switch between them with Ctrl+1, Ctrl+2, and Ctrl+3.
In Anki, templates are written in HTML, which is the language that web pages are written in. The styling section is CSS, which is the language used for styling web pages.
On the right is a preview of the front and back of the currently selected card. If you opened the window while adding notes, the preview will be based on the text you had typed into the Add Notes window. If you opened the window while editing a note, the preview will be based on the content of that note. If you opened the window from Tools → Manage Note Types, Anki will display each field’s name in parentheses in place of content.
At the top right of the window is an Options button that gives you options to rename or reorder the cards, as well as the following two options:
The 'Deck Override' option allows you to change the deck that cards generated from the current card type will be placed into. By default, cards are placed into the deck you provide in the Add Notes window. If you set a deck here, that card type will be placed into the deck you specified, instead of the deck listed in the Add Notes window. This can be useful if you want to separate cards into different decks (for instance, when studying a language, to put production cards in one deck and recognition cards in another). You can check which deck the cards are currently going to by choosing Deck Override again.
The 'Browser Appearance' option allows you to set different (perhaps simplified) templates for display in the Question and Answer columns of the browser; see browser appearance for more information.