We have, alas, several certificate types in Tor.

The tor_x509_cert_t type represents an X.509 certificate. This document won't explain X.509 to you – possibly, no document can. (OTOH, Peter Gutmann's "x.509 style guide", though severely dated, does a good job of explaining how awful x.509 can be.) Do not introduce any new usages of X.509. Right now we only use it in places where TLS forces us to do so. See x509.c for more information about using this type.

The authority_cert_t type is used only for directory authority keys. It has a medium-term signing key (which the authorities actually keep online) signed by a long-term identity key (which the authority operator had really better be keeping offline). Don't use it for any new kind of certificate.

For new places where you need a certificate, consider tor_cert_t: it represents a typed and dated something signed by an Ed25519 key. The format is described in tor-spec. Unlike x.509, you can write it on a napkin. The torcert.c file is used for manipulating these certificates and their associated keys.

(Additionally, the Tor directory design uses a fairly wide variety of documents that include keys and which are signed by keys. You can consider these documents to be an additional kind of certificate if you want.)