As of today, Twitter apps will all use OAuth for user authentication. Users will be able to use apps without them storing their password.
"The move to OAuth will mean increased security and a better experience. Applications won’t store your username and password, and if you change your password, applications will continue to work," says Twitter's Carolyn Penner says, "With OAuth, you still individually approve each application before using it, and you can revoke access at any time."
"In order for Twitter applications to access your account, developers have been able to choose one of two authentication methods: Basic Authentication or OAuth," says Penner. "Both require your permission, but there is an important difference. With Basic Auth, you provide your username and password for the app to access Twitter, and the application has to store and send this information over the Internet each time you use the app. With OAuth, this isn't the case. Instead, you approve an application to access Twitter, and the application doesn’t store your password."
A lot of Twitter users are already using apps that use OAuth. Echofon, TweetDeck, Twitterrific, Seesmic, and Twitter for Android, iPhone, and Blackberry already use it.
Twitter users can go to the "Connections" section under settings and see what all apps they've authorized and to revoke access if necessary. If you're not using the latest versions of any apps, they may stop working because of the change.