On July 1st, 2009, Twitter raised the REST API rate limit from 100 "GET" requests per hour to 150 "GET" requests per hour.http://apiwiki.twitter.com/REST-API-Changelog

The REST API allows access to user timelines, user information, status information, and other core data-sets from Twitter. The REST API uses a hourly rate limit for GET requests. You can apply for whitelisting if you need more than the current rate limit. The current whitelisting limit raises the 150 requests per hour limit to 20,000 requests per hour. These limits do not apply to POST requests, which are used for creating tweets and direct messages. Different limits apply for non-GET requests.

The Search API allows access to Twitter Search. The Search API is entirely separate from the REST API, meaning user IDs may be different than those in the REST API. Data is available through Atom and JSON formats.

The Streaming API allows access to the Twitter "firehose" and other large continuous streams of Twitter data. The Streaming API is often used for analytical purposes. Some methods in the Streaming API require that you sign a legal document with Twitter about how you may use the data retrieved from the Streaming API.