Most of API requests requires authentication based on application name and application token. To obtain them you have to register your application on Application page.
After credentials generation you could use them in your applications by adding HTTP headers X-Auth-Application and X-Auth-Token to your requests:
GET /api/v1/object/list.json
X-Auth-Application: myapplication
X-Auth-Token: mytoken
...
NOTE: Some API handlers designed to use cookie-based user authentication or both of token-based and cookie-based.