To display certificate information of a certificate issue the command below:

openssl x509 -in certificate -text

Information about the certificate is displayed. Some important items are:

Issuer: C=GB, ST=Greater Manchester, L=Salford, O=COMODO CA Limited, CN=COMODO RSA Domain Validation Secure Server CA

The Issuer is a CA that signed this certificate.

Validity
            Not Before: Feb  6 00:00:00 2015 GMT
           Not After : Feb 26 23:59:59 2016 GMT

The validity period of the certificate. Remember to renew your certificate before it expires!

Subject: OU=Domain Control Validated, OU=PositiveSSL, CN=www.bjdejong.nl

The subject for this certificate. This certificate can be used for a website for the given CN.