SHA – standing for Secure Hash Algorithm – is a hash algorithm used by certification authorities to sign certificates and CRL (Certificates Revocation List).
SHA-1 is an older version of the algorithm that is no longer considered to be secure by major browsers and industry experts. SHA-1 is no longer allowed to be used during the generation process by the industry.
SHA-2 is the latest version that is widely accepted and considered to be secure by all major industry experts and browsers. The encryption hash used in SHA-2 is significantly stronger and not subject to the same vulnerabilities as SHA-1.