Due to a rash of spam account registrations (i.e. scripts trying to register for the site so they can post spammy comments on posts and pages), we have added a reCAPTCHA to the login and registration forms. This will appear to you as shown here.
Simply click the box to confirm that you are a human. In some cases (for example, your IP appears to have performed suspicious activity) you may be asked for further "easy for a human" activities such as identifying the contents of photographs.