Autofill in Web browsers has been a beneficial feature for whenever you encounter a Web form where you need to enter your name, address, phone number, and other details being requested. While autofill generally stores or accesses your address and contact information, it may not store your credit card details, which are often required when filling out billing information on your computer.
To enter credit card information, numerous online services have options for creating an account and then entering credit card information there; however, this needs to be done for all places that you shop.
If you use Safari in OS X Mavericks, then one option available to you is Apple's Secure Credit Card storage, where you store information about your card locally in Safari. When you encounter an online form that requests credit card information, Safari will prompt you to choose from among your stored credit cards, to enter information from. This information includes the card number, the full name on the card, and the expiration date.
To have Safari store credit card information, go to the Autofill section of Safari's preferences and check the box next to Credit Cards. Now, whenever you enter new credit card information in a Web form, Safari will prompt you to save it. Alternatively, you can click the "edit" button next to the credit card toggle in Safari's preferences, and can add, remove, or edit your card information that way.
Note that while this does make entering credit card information more convenient, it still does not store your card's CVV code, which is often used as an extra requirement when entering card information. Therefore, you may still need to keep your credit cards handy even when you use Safari's autofill information, but will at least only need to enter this number instead of the entire card profile.