In addition, we’ve described a way that people will be able to download copies of our certificates.
https://letsencrypt.org/certificates/
This is in addition to our OCSP responder, which lets people check whether the certificates have been revoked. In the future, we might have more APIs like this as well.
But as @kelunik says, the main API is our ACME endpoint, which is specifically designed to automate the whole issuance (and revocation) process and allow interoperability with third-party software for these purposes.