Overview of Cert filetype

PEM
This is a (Privacy-enhanced Electronic Mail) Base64 encoded DER certificate, enclosed between “—–BEGIN CERTIFICATE—–” and “—–END CERTIFICATE—–“
openssl req -newkey rsa:2048 -new -nodes -x509 -days 3650 -keyout key.pem -out cert.pem
CER & CRT & DER
Although usually in binary DER form, Base64-encoded certificates are also common (see .pem above).
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P7B & P7C
PKCS#7 SignedData structure without data, just certificate(s) or CRL(s)
P12
PKCS#12 files may contain certificate(s) (public) and private keys (password protected).
PFX
PFX is the predecessor of PKCS#12. This type of file usually contains data in PKCS#12 format (e.g., with PFX files generated in IIS).
openssl pkcs12 -export -out certificate.pfx -inkey privateKey.key -in certificate.crt -certfile more.crt

Openssl Tutorial & Cert Convert

Convert PEM to DER
openssl x509 -outform der -in certificate.pem -out certificate.der
Convert PEM to P7B
openssl crl2pkcs7 -nocrl -certfile certificate.cer -out certificate.p7b -certfile CACert.cer
Convert PEM to PFX
openssl pkcs12 -export -out certificate.pfx -inkey privateKey.key -in certificate.crt -certfile CACert.crt

Convert DER to PEM
openssl x509 -inform der -in certificate.cer -out certificate.pem

Convert P7B to PEM
openssl pkcs7 -print_certs -in certificate.p7b -out certificate.cer
Convert P7B to PFX
openssl pkcs7 -print_certs -in certificate.p7b -out certificate.cer
openssl pkcs12 -export -in certificate.cer -inkey privateKey.key -out certificate.pfx -certfile CACert.cer

Convert PFX to PEM
openssl pkcs12 -in certificate.pfx -out certificate.cer -nodes