There was a time when getting the right email address in Yahoo was never possible, what with people using ids such as yourname1234 and yourname_surname32432 and many such ids which were literally hard to remember.
The reason for such ids being that Yahoo has been there for quite a while and almost every sane email id has been taken over. Yahoo on the other hand had been getting quite a lot of complaints from users who are not able to signup with the ids they want.
To tackle this issue Yahoo has rolled out two new domains ymail.com and rocketmail.com. Nothing has changed except for the new domains, you will still benefit from all the features the regular Yahoo mail provides you with.
So if you have been waiting for that perfect email address with Yahoo head off to http://new.mail.yahoo.com/addresses and get signed up right now before your perfect email address is taken by someone else.