There are many ways a program can express interest in you
Verbal statements are impressive but non committalYour interview went stellar, everything worked in your favor and you know that you made the right choices. You have a good feeling about the interview. you know it and the program director knows it. Very often, they will have 'exit interviews' where they will tell you how you have fared. They will often express interest in you. Since there is no commitment in writing, you can get a very frank assessment of your chances.
Applicant specific post interview noteRarely, The program will send out an email that is tailored for the applicant. This is very good sign. They will say that you have an excellent chance and word their letter in such a way that says everything short of "we will rank you in the top 5". These are exceptionally promising. no PD is going to tell you with guarantee about any ranking prior to the match or else they will get into trouble. Hence they will use 'non-liable' language.If you are lucky to get a letter like that and you like the program, it may be worthwhile to ask for out of match offer. Check this link about asking for prematch.
Generic post interview noteThis is the most common form of feedback applicants receive after the interviews. The letters are mass mailed to almost everyone who shows up for the interview. They do not mean much other than completion of a formality. To many applicants, they offer a glimmer of hope or dash of dejection. They are very bland and not useful. A way to identify these emails is general usage of words, non specific details, no mention of applicant specific characteristics. One good thing about these emails is they may provide the email address of the program director. Check this link about finding email addresses of program directors.