listener.ora on the server side.
tnsnames.ora on client side with connect string pointing to the related listener entry.
tnsnames.ora is searched for the server information but before that sqlnet.ora is read to find out the sequence of search that is whether it should first look in tnsnames.ora file, names server etc.
Also if Advanced Networking is option is installed then parameters related to that are matched with the server sqlnet.ora
Also the default domain is taken from sqlnet.ora .

Thanks
GP