The Gnip API provides notification of activities (events) occurring in a variety of services and, whenever possible, a guid that identifies the activity itself vis a vis the service it was created on. Activity examples include a user "tweet" (twitter), a user "dugg" (digg), a user creating a blog post, etc.
There are two primary roles that API users fall into; Publishers and Subscribers. You may be one, the other, or both depending on your situation. Publishers push data into the system and subscribers consume data from the system.
Gnip also provides access to Twitter firehoses for non-display purposes, sometimes with a fee.