Getting Information About My Organization | REST API Developer Guide

List Available REST API Versions
Use the *** resource to list summary information about each REST API version currently available, including the version, label, and a link to each version's root. You do not need authentication to retrieve the list of versions.

List Organization Limits
Use the Limits resource to list your org limits.

List Available REST Resources
Use the Resources by Version resource to list the resources available for the specified API version. This provides the name and URI of each additional resource.

Get a List of Objects
Use the Describe Global resource to list the objects available in your org and available to the logged-in user. This resource also returns the org encoding, as well as maximum batch size permitted in queries.

Get an Image from a Rich Text Area Field
Use the SObject Rich Text Image Retrieve to retrieve an image that has been uploaded to a rich text area field.

Get a List of Objects If Metadata Has Changed
Use the Describe Global resource and the If-Modified-Since HTTP header to determine if an object’s metadata has changed.