.cer
Used for binary certs
CER or DER
.pem
.p7c
BINARY CER
.pfx
.der
.crt
server cert, cert chain, keys, CRL
certs, cert chains, CRL
_,cer
DER format
ASCII
Never private key
contents vary
Used to store private key with a cert
.p7b
Any purpose
Used to share public key
.p12
.key
Used on windows system
Certs, cert chains, private keys
BINARY
Used for Ascii certs
