OpenSSL is a full featured tool capable not only to generate keys and certificates, but also to provide every facility a PKI must have, such as indirect CRL and OCSP responders: these features, along with certificate's best practices such as the Certification Practice Statement (CPS), publishing CRL Distribution Points URL, OCSP Responders URL, CA Issuers URL, are the topics of the OpenSSL CA tutorial - A full-featured openssl PKI.
As we saw in our post on Symmetric And Asymmetric Cryptography, asymmetric key pairs can be used to encrypt and digitally sign documents, but have a huge shortcoming: since they are just keys, they don't provide the metadata necessary to enable people to securely identify their owner. As we saw in the post on GNU Pretty-Good Privacy, GPG addresses this shortcoming by wrapping the key into a container object (the GPG key) that also embeds some metadata about the owner of the key. Of course also the IETF addressed this problem, defining the standard for X.509 certificates (currently X.509v3). X509 Certificates Howto & Public Key Infrastructure Tutorial explores X.509 certificates and how they are trusted by the means of a Public Key Infrastructure.