This is one of the best acronyms in all of geekdom: "What You See Is What You Get". Most cards have a "wysiwyg" editor, which basically looks something like this (learn about using the wysiwyg editor):
Saving/cancelling and deleting cards
Underneath the editing area, you'll see "Save" and "Cancel" buttons. Click "Save" to save your edits, or "Cancel" if you want to leave the card as it was. If you have permissions, you'll also see a button for deleting the card.