Twitter is sending out messages to a number of users requesting that they change their passwords.

It seems that they have detected some unusual activities in a number of accounts and they suspect fraudulent use. It appears that someone has been creating malware-laden Internet services and collecting usernames and passwords. Since people have a bad habit of using the same username and password for different services, so credentials gathered in one attack can be used to attack another service (in this case Twitter).

The message is to update your Twitter password if you receive a notice or suspect your account could be compromised (go directly to https://twitter.com), and use a separate, strong password for each service you use.