What is inside an SSL certificate?

An SSL certificate usually contains a name, public key, the digital signature of the certificate issuer, the name of the issuer, certificate serial number, expiration date and additional information.

X.509 Version 3 certificates support the following fields:
Subject: Provides the entity details that the CA issues the certificate to.
Issuer: Provides a distinguished name for the CA that issued the certificate.
Validity: Provides the date and time when the certificate becomes valid, and becomes invalid. The date when an application evaluates the certificate must fall between the Valid From and Valid To fields.
Serial Number: Provides a unique identifier for each certificate that a CA issues.
Public Key: Contains the public key of the key pair that is associated with the certificate
Signature Algorithm: The algorithm used to sign the certificate, like SHA-256 with RSA Encryption
Signature: Bit string containing the digital signature, like 512 bytes : 0D 60 34 91 79 92 CE 50 AE xx xx .....
Fingerprints: Provides the unique identifier of the certificate. In X.509-based PKI system, fingerprints are primarily used to authenticate root keys. 

X.509 version 3 certificates also include optional extensions:
Subject Alternative Name: This extension allows multiple hostnames to be protected by a single certificate. SAN certificate is also known as Unified Communication Certificate (UCC) or a multi-domain certificate.

CRL distribution points (CDP): When a server presents a certificate, an application or client must determine whether the certificate has been revoked. The CDP extension provides one or more URLs where the application or service can retrieve the certificate revocation list (CRL) from.

Authority Information Access (AIA): When validating a certificate, the certificate of the CA that issued the certificate, also referred to as the parent CA, must also be evaluated for revocation and validity. This extension provides one or more URLs from where an application can retrieve the issuing CA certificate.

Enhanced Key Usage (EKU): This attribute includes an object identifier (OID) for each application  a certificate can be used for. Each OID is a unique sequence of numbers from a worldwide registry.

Certificate policies: Describes what measures an organization takes to validate the identity of a certificate requestor before it issues a certificate. An OID is used to represent the validation process and can include a policy-qualified URL that fully describes the measures taken to validate the identity.