Guerrilla Mail is an anti-spam solution, where a user is given a temporary Email address which they can use without the need of disclosing personal information. Users can also subscribe to Guerrilla Mail and receive a permanent Guerrilla Mail address (although mail sent to that address will only be stored for an hour). The Guerrilla Mail API is a JSON based HTTP API that provides all the functionality to create a fully featured app. Developers can use the API to create their own Guerrilla Mail website or application, or integrate Guerrilla Mail with their own applications.
Guerilla Mail is a simple idea with one purpose: to avoid spam. The idea is dead simple: go to the website, it gives you a random e-mail address that's good for one hour, and links to nothing else. Register for sites or whatever with it, and then never worry about it again. Although the web service is convenient, sometimes it's not everything you need. For those cases, one might want to look into using the Guerilla Mail API.