The title really says it all. Last year, Twitter started giving its users the option to use HTTPS to keep their connections safe over unsecured Internet connections. Today, the company announced that it is now making secure SSL connections the default for all users.
HTTPS ensures that the traffic between the server and your browser is encrypted and can't easily be intercepted over unencrypted wireless networks, for example. This is essentially the same protocol you use when you access your online bank accounts, for example.
With this move, Twitter is following in the footsteps of other companies like Google, which made HTTPS the default for all Gmail users in January 2010 and for all signed-in Google Search users in late 2011. Facebook, too, users HTTPS whenever a password is sent to the service, but users have to manually activate secure connections for all of their other activity on the service.
In addition to adding these secure connections to Twitter.com, the company also announced that it plans to improve HTTPS support on its web and mobile clients in the future.