A X.509 certificate is a structured grouping of information about
an individual, a device, or anything one can imagine. A X.509 CRL
(certificate revocation list) is a tool to help determine if a
certificate is still valid. The exact definition of those can be
found in the X.509 document from ITU-T, or in RFC3280 from PKIX.
In OpenSSL, the type X509 is used to express such a certificate, and
the type X509_CRL is used to express a CRL.
A related structure is a certificate request, defined in PKCS#10 from
RSA Security, Inc, also reflected in RFC2896. In OpenSSL, the type
X509_REQ is used to express such a certificate request.
To handle some complex parts of a certificate, there are the types
X509_NAME (to express a certificate name), X509_ATTRIBUTE (to express
a certificate attributes), X509_EXTENSION (to express a certificate
extension) and a few more.
Finally, theres the supertype X509_INFO, which can contain a CRL, a
certificate and a corresponding private key.
X509_..., d2i_X509_... and i2d_X509_... handle X.509
certificates, with some exceptions, shown below.
X509_CRL_..., d2i_X509_CRL_... and i2d_X509_CRL_...
handle X.509 CRLs.
X509_REQ_..., d2i_X509_REQ_... and i2d_X509_REQ_...
handle PKCS#10 certificate requests.
X509_NAME_... handle certificate names.
X509_ATTRIBUTE_... handle certificate attributes.
X509_EXTENSION_... handle certificate extensions.