I knew that all Digital certs confirm to the X.509 standard. But why were they so many extensions for certificate files? - e.g. DER, CER, .P7B, PFX etc. I found this link that contains good info about X.509 certs.
Basically a X.509 cert can be in many formats (depending on the encoding used to save the cert)
.CER - CER encoded certificate, sometimes sequence of certificates
.DER - DER encoded certificate
.PEM - (Privacy Enhanced Mail) Base64 encoded DER certificate, enclosed between "-----BEGIN CERTIFICATE-----" and "-----END CERTIFICATE-----"
.P7B - See .p7c
.P7C - PKCS#7 SignedData structure without data, just certificate(s) or CRL(s)
.PFX - See .p12
.P12 - PKCS#12, may contain certificate(s) (public) and private keys (password protected)
A .P7B file could be a container for more than one digital certificate.