Twitter has announced that they have 300,000 registered applications on OAuth API. Oauth is an API standard which is secure and widely acceptable now a days.
Initially twitter launched its non Oauth API which was referred as basic API. Twitter launched its OAuth based API in 2009 and till this August , they were continuing with both the APIs. However in April twitter has confirmed that they will shut down the services from their basic API, and promote and use only OAuth based API.
Twitter API has played a major role in the popularity of twitter, many third party developers have developed big and successful traffic resources like Twitpic,Tweetdeck and Uber Twitter for Twitter.