What is an issuer?
An issuer is an Certificate Authority that signs and issues certificates. It will vouch for the certificate it signs as genuine. Only a valid issuer can properly sign certificates. A self-signed root Certificate Authority signs and issues itself a certificate. A intermediate Certificate Authority can be signed by another a signed-signed root CA or an intermediate CA. This chain of certificates is known as a certification path or certificate chain.  

What is a certificate chain?
A certificate chain is composed of a root certificate at the top, followed by any intermediate certificates, ending up with the leaf certificate at the bottom. A certificate chain at a minimum contains a root and a leaf and no intermediates.

What is an intermediate Certificate Authority (intermediate certificate)?
A intermediate CA issues certificates and is signed by another intermediate CA or self-signed root CA. 

What is a leaf certificate?
A leaf certificate is the last certificate in the certificate chain. It is the certificate actually being used by the user to perform operations.